[*IsBack]

Some function in returning 0 (instead of 1 or anything else). Returning 0 on OnPlayerCommandText callback, gives you "Unknown command" error. One of your filterscripts may cause this problem too..
Make sure to return 1; after every command check (strcmp), and make sure you got return 0; (or return SendClientMessage(playerid,color,"Unknown command!!"); ) at the end of OnPlayerCommandText callback.
Edit: I'm feeling ignored....