Simply put, I joined a co-op team and we were doing normal runs starting at the beginning. We all readied at 'The Wizard's Workshop' (level 2) but it wouldn't load and kept the "Waiting for team" screen. We tried to disband the party, thisisacat invited us again, and the same problem occurred.
Did you try changing the party leader? If this problem persists, email support@cardhunter.com to get it sorted out.