This is an old issue others have had in the past and it's resurfacing - I've looked up several solutions, and now need to find even more because none of the following seem to work;

Restarting steam
Reinstalling the game (And deleting ALL existing game files while doing so)
Verifying the integrity of the game cache
Installing compatability modules for older windows versions, as suggested by FAQ post (I'm running W10)
SKIPINTRO and NoStartUp commands to the launch options
(Replaced Launcherexe with the AoE2exe for the NoStartUp)
Deleting the movies from the gamefolder
Launching the game directly from folder (Leads to another critical error)
Opting in/out of betas that may affect it
THE FIX; Ensuring Anti-Virus software wasn't a ♥♥♥♥ and blocking it, which it was. I had to add the main .exe file of AoE2 HD to an exception list.
Anyone else got this problem AND managed to fix it?