Without version 1.3.1, open co-op mods will either crash on launch or refuse to sync player animations. If you own the game on Steam or GOG, you likely already have 1.3.1. However, the original CD release requires manual patching.
To ensure playability, you must use the original PC/Steam version of Doom 3 (not the BFG Edition). : Ensure Doom 3 is updated to patch 1.3.1 . Without version 1