There's currently an issue with AMX Mod X (I believe this occurs with regular AMX as well) where changing a map with "amx_map" or the maps menu will result in clients being locked out of their join team menu.
Here is a fixed version of the latest nightly AMXx build. Any version after this will also contain this fix.
If you know of any other incompatibilities of AMX Mod X 0.20 with Zombie Panic please let me know and I'll try to fix them ASAP.