Same thing been happening to me lately. I am using MB2 Engine.
I can still join a specific server by typing /connect
For now try using this:
Movie Battles II Server List
(copy chosen IP) and type it in your console (shift+tilde to open) along with connect, for example:
/connect
193.124.182.152:29070
One thing that is triggering this fenomen for sure is instalation of a new mod/addon - removing that addon/mod will solve the issue and re-installing it will solve the problem itself. I am using mods from our forum, I wanted to replace sounds within the modification with my own, once I did that server list was blank, but as I said before I could still connect using /connect command. Strange.