Hello, i am also a linux proton experimental user who can no longer login to any servers. I get the 000200009 error every time. This is 1.24, as of Feb. 26th after downloading steam proton updates this morning. Played all day yesterday and had a fine time, today it is broken.
Edit, Proton 8.05 worked and i am able to join a server.

I have the same problem, stuck under the new building in Tisy. This method did not work for me as Steam gives "corrupted files" warning upon logging in and quits.

By deleting the structure_tisy_base.pbo file and logging in,  running, logging out I was successful getting unstuck. I then just verified files in steam and logged in.