I can't join servers now because, supposedly, the level that was downloading has the wrong CRC. I don't understand. What does this mean?
Can I do anything to get rid of this?

probably the person hosting saved the level without restarting or rehosting.

This also happened to me before I tried to join that server today, like say, on the battle server, (a long time ago) sometimes it would do that, sometimes it would also say "No start positions found! This is an internal error and cannot be solved."