When I join a server (sometimes) there are squares instead of the map picture.

How to fix it?

Uh, the .iwd of the 'map' is broken, redownload it.

I'm sorry for not giving you the full explenation. Sometimes it is the picture of the map and sometimes there are squares. example map Rust

Follow this tutorial: viewtopic.php?f=26&t=8457 and re-download the missing/corrupted files.

They are not corrupted, ok?

 

Well, seems that it just randomly happens, maybe when the .iwd didn't load into memory yet? Try staying in main menu a bit longer and try launching a map again.