Look in the server's user.ini (not server.ini or whatever) for this section:
Code:
[DefaultPlayer]
Name=Some_Twat
Class=BotPack.TMale2
team=0
skin=SoldierSkins.sldr
Face=SoldierSkins.Brock
Voice=BotPack.VoiceMaleTwo
The class and skin you enter here is what players will end up with if they attempt to use a skin that is not installed on your server. If you dont specify anything, your server will crash in stead.