I have a hunch this is caused by the auto-switch feature. When you click on a client, it tries to make that computer the server. Sometimes the other machines are unable to connect to this new server, hence the progress bar. The UI is also very temperamental at the moment, so auto-switching a computer back and forth to server and client mode can cause some weird UI bugs.
If the UI gets into a weird state, try closing Synergy on every computer, then start Synergy back up again.
@Andrew Nelless and @Jerry Hou will be fixing the auto-switch feature and also the UI bugs in beta4 (which is