Workshop should deliver all the files but if it is down or acting up and the player is not subscribed to the mod, then I think this is when the model errors happen.

There is a backup built in for this by using the sm_downloader sourcemod plugin but I found that there is an error in the config.

In the server files go to insurgency/addons/sourcemod/configs and locate and open downloads.ini

find the line: models/ins_zombie and change it to models/characters/ins_zombie Save the file, close and restart the server.

Let me know if this fixes some of the error problems