Either:
1. You have a conflicting mod that overwrites Skyrim Unbound plugin/scripts, namely qf_mq101_0003372b.pex script or/and MQ101 "Unbound" [QUST:0003372B] record.
2. You have not enabled the mod in your mod-manager. For example, in Mod Organizer you need to enable the mod on the left panel and also activate the .esp plugin on the right panel.
3. You did something else wrong when installing the mod or managing it in your mod-manager.

- Infinite loading (very unlikely for CTD) can be caused by a conflicting mod that overwrites Skyrim Unbound plugin/scripts, namely qf_mq101_0003372b.pex script or/and MQ101 "Unbound" [QUST:0003372B] record.
- Infinite loading can also be caused by Skyrim Unbound.esp being disabled while the other files of the mod are still enabled in your mod-manager.
- Infinite loading on Skyrim v1.6.1130+ can be caused by Engine Fixes. Some of its settings enabled by default are incompatible with these versions of Skyrim, in particular EnableAchievementsWIthMods prevents all your .esp-s from being loaded into the game, which causes the issue mentioned above. All the info about this can be found in posts on the Engine Fixes page, or you can use a pre-configured .toml file.
- Also make sure that the .esp-s of all Skyrim Unbound patches and addons are placed lower than the main Skyrim Unbound.esp

If nothing of this is the case, most likely your issue is not related to Skyrim Unbound. Some possible cases:
1. CTD, freeze or infitnite loading on game start caused by some other mod. In this case the issue will remain even if you disable or uninstall Skyrim Unbound (not only the .esp but all files) and use any other alternate start mod or vanilla start.
2. CTD, freeze or infinite loading in the Helgen area caused by some other mod that changes, adds or affects something in this area. In this case the game will run successfully if you set quick start to 1 in SkyrimUnbound.json (works only if you have PapyrusUtil installed and working), which makes it spawn you in the Skyrim Unbound starting room instead of Helgen, and you will likely have the same issue with vanilla start or if you just visit this area after the start.
3. If you use Skyrim from GOG or Epic Games with Mod Organizer 2, make sure that you use the latest version of Mod Organizer 2. Versions before 2.5.0 didn't work with GOG and Epic Games, preventing the game from starting.

They're hidden on purpose. If you're interested in details, check FAQ (Features > Why hide the mod messages at game start at the left top corner of the screen?).

This option is added by SkyUI, which is required for SUR. If you don't have that option, then either you don't have SkyUI installed or it's not working. The latter may happen if your SKSE is not working (maybe the version doesn't match your Skyrim version) or you don't have it installed at all (SKSE is required for SkyUI). You may also have a mod that renames the "Mod Configuration" option to something else ("MCM") or changes the order of the options in the menu.

The MCM is named "Skyrim Unbound". It may be the first one in the list (by default) or in the alphabetical order (if the "MCM On Top" global setting is disabled).
If you have a lot of mods with MCMs, it may need some time to appear. Some mods can slow down the loading of MCMs even more, for example iNeed is known to to that.

There were reports that Poser Hotkeys Plus when used with Precision causes that (you can disable Poser Hotkeys Plus and enable it back after the game start). You can also try to enable Quick Start in global settings and see if that will prevent the death.

Some other mod forces third person at the game start. Combat Fatigue is known to do that (better alternatives: EVG Conditional Idles, Injured Animations Dynamic Replacer).

You have a conflicting mod that overwrites Skyrim Unbound plugin/scripts.

You use unsupported characters in character the name. Skyrim Unbound doesn't affect that in any way.

This crash is caused not by Skyrim Unbound, but by something in the area you're being teleported to that is added, changed or affected by some mod that you have. Try to visit this area in a different way and see if the game crashes. You can try multiple times in different times of day if it doesn't crash on the first try, because there are randomized objects and events, NPC schedules, etc.

If you're working on a location addon, you probably did something wrong.
Otherwise most likely you have an unmatching skyrimunboundquestscript.pex/psc and SkyrimUnbound quest record. If you have Skyrim Unbound - Fixes or the Experience patch from its page, delete them because they're incompatible and not needed with Skyrim Unbound Reborn (except for UHDAP voices, you can get only them from its optional files if you prefer them over the default LE-quality voices). If you don't have it, you may have remains of an older version of Skyrim Unbound Reborn or the original Skyrim Unbound overwriting the latest version of Skyim Unbound Reborn.

You have a conflicting mod that overwrites Player "Prisoner" [NPC_:00000007] record from the Skyrim Unbound.esp.

Notice that the delay has two stages. After the first stage only dragons at word walls, such as Bonestrewn Crest (jn the middle of volcanic tundra), get enabled. After the second stage dragons start to appear randomly in the world as well. In Timed mode each stage takes a random number of days between "Min Days" and "Max Days" set in the MCM. In Leveled mode each stage starts on the corresponding level.

Notice that (except when you set Dragons to Enabled or Disabled in the MCM) the dragons don't get enabled immediately after you change dragon settings or when the time passed/the level is passed, you need to change location to make SUR update dragons (for example travel to any city).

Check if SUR has already enabled dragons:
prid 5eacd
getdisabled
If it displays 0 - it already enabled word wall dragons.
If it displays 1 - it haven't enabled them yet (if the required time/level has already passed you need to change location to make SUR update dragons).
GetGlobalValue DragonsReturned
If it displays 1 - it already enabled random dragons (if there are still no dragons, the time just hasn't come yet or something prevents them from spawning).
If it displays 0 - it haven't enabled them yet (if the required time/level has already passed you need to change location to make SUR update dragons).
(It's however technically possible for other mods to disable both types of dragons in the same way as Skyrim Unbound, for example during some quest, since these are vanilla values used to control dragon spawns)

Things that can prevent random dragons from spawning in the base game if they're already enabled:
1. "A Blade in the Dark" chapter of the main quest active;
2. The vanilla cooldown between dragon spawns: between random dragon spawns, and it seems that encountering a word wall dragon activates the same cooldown too, preventing random dragons from spawning for some time. Enter in console "GetGlobalValue WINextDragon" and then "GetGlobalValue GameDaysPassed": if GameDaysPassed is less than WINextDragon then the cooldown is not expired yet and you need to wait the remaining time (the difference between the two values, which will be in game days).

If dragons are not enabled when they should, you can change the Dragons MCM option to a different value and back, or just set it to Enabled.

Check if SUR has already enabled dragons:
prid 5eacd
getdisabled
If it displays 0 - it already enabled word wall dragons. If it displays 1 - it haven't enabled them yet.
GetGlobalValue DragonsReturned
If it displays 1 - it already enabled random dragons. If it displays 0 - it haven't enabled them yet.
(It's however technically possible for other mods to enable both types of dragons in the same way as Skyrim Unbound, for example after some quest during which it disables them, since these are vanilla values used to control dragon spawns)

If it hasn't enabled any dragons yet, but you still encountered some dragons, these dragons are not vanilla dragons but dragons spawned by some mod you have that implements its own custom dragon spawn system that doesn't check vanilla variables that control dragon spawns and are used by Skyrim Unbound to delay dragons.

The "Dragonborn" MCM option defines if you're a dragonborn or not, and it's set to Random by default. You can safely change it after the start in the MCM.

After you enabled it, you can't absorb souls of already killed dragons, you'll have to find a new one.
Also, it was reported that soul absorbing doesn't work on dragons spawned via console.

You have a conflicting mod that overwrites Skyrim Unbound plugin/scripts.

The Greybeards only call you after you absorb your first dragon soul. There's also a 35-sec delay between the soul absorbing and the call.
If this time already has passed but nothing happens, you most likely have a conflicting mod that overwrites Skyrim Unbound plugin/scripts.

It's not affected by SUR, but you need to move from one location that is not Whiterun to another location that is not Whiterun for the quest to advance. That's how that quest works and always worked.

It's a common compatibility issue with Skyrim Unbound. It happens due to conditions like "if MQ104 (Dragon Rising) quest is completed" that other mods use to check if you're a thane of Whiterun, when Skyrim Unbound completes Dragon Rising when you absorb your first dragon soul. So the issue is most likely caused by some mod that uses such conditions.
You can use my Find Dialogue Info xEdit script to find a particular dialogue record by a fragment of its text in xEdit. For info about patching such conditions see Patching conditions for Skyrim Unbound.

1. Make sure that you have PapyrusUtil installed and that it works. You can check the latter by going to the first page of the Skyrim Unbound MCM and checking if there are "Save Preset" and "Load Preset" options. If there are no such options, it means that your PapyrusUtil doesn't work and you should check the next issue in the list.
2. Addons are only loaded when you start a new game from the main menu. So, if you installed an addon and can't find it on your old save, start a completely new game.

Make sure that nothing overwrites PapyrusUtil. Some mods (including Campfire and Simply Knock) include a copy of an outdated version that doesn't work now - remove or hide SKSE\Plugins\PapyrusUtil.dll from them or just let latest version of PapyrusUtil overwrite everything.
Also make sure that the installed version of PapyrusUtil matches your game version.