having the same problem. init thought it was my better keep corpses addon but after thorough experimentation its not the issue for me. if keeps corpses are off you taht is probably why.

another thing: my corpses are fading instantly, so the ragdolls arent animating into zombies. will try to fix via more experimentation

Can you please elaborate on what you mean by fading?

when i kill zombies, after some time their corpses are disappear

That is normal; it's a performance saving feature. You can turn it off by setting corpse life to -1 under Miscellaneous settings.

YOU STUPID ♥♥♥♥ YOU DONT HAVE KEEP CORPSES ON!!!!

BA2 corpses behave the same regardless of Keep Corpses. Be calm and make sure you know what you're saying before you say it.