New installs of chivalry cannot join modded servers, and new modded chivalry servers cannot be joined.
This must be a problem with chivalry itself. For some, deleting cookedsdk, validating files, and starting through the launcher has worked to solve the problem of not being able to join working modded servers. I do not know of a fix for the new modded servers, because even if you can join the old modded servers, you cannot join new ones, like the one I had recently paid for.
I just wanted to add that this is a problem with everyone I know who has recently reinstalled chivalry.