When joining a CoScreen, the UI is stuck in the greyed out joining phase, but I never receive the "Unable to connect to CoScreen" dialog.
CoScreen currently expects to live on a single, non fixed, desktop. If you have configured the application to reside specifically on one or more desktops this problem will occur.
To resolve this issue right click the CoScreen application icon, choose options, and ensure that "Assign To" is set to "None". Click "Leave" and rejoin the CoScreen.