KPS

I've been playing on a server. Last night, a left the game normally. Now, I got on normally as well, and now I can't join and it says "this save is incompatible l. Please switch to steam branch "build23" to continue this save." The worst part is that I had a LOT of good stuff on my character. I don't own the server, and the server owner doesn't know how to fix this, so please help.