I have managed to narrow down the error to the extraweapons.hud itself (I removed it then it worked then re-added it and got the same crash and error).
Only interesting errors in my error log (the com_inv_mesh one I've had on both this map and the other test map):
Hidden/Spoiler:
When I re-added the Extraweapons.hud it was the default one straight from the downloaded tutorial to fix it by Fragme, so no need to post that.
Any help appreciated, it truly would be a waste for so many people's work to be stuck with this annoying bug.

