Me and my friends have been having the same issue on zombies for a few days now. Seems to be only on a select few maps, as we can join each other on Nuketown, but we get the error on Origins.
EDIT: Found a thread with a few fixes -
Chase said in Incompatible UI:
UI Compatibility Error
Since the update of r2107 people have been getting this error in T6 Zombies. This only occurs when players attempt to join their friends via custom games in DLC maps. A fix will be rolled out soon, please be patient!Possible solutions to try in the meantime:
Join at same time - You can use console command
zombies_minplayer xwhere x = amount of people required to join in order the game to start. You must type this before starting the match. Hopefully this will prevent the error.Joining a public server playing those maps - at the moment, joining servers does not cause the error to occur and yes Easter Eggs work in servers.
(for advanced users only) Host a public server - take a look at this guide to help you https://plutonium.pw/docs/server/t6/setting-up-a-server/