Archicad must be able to communicate with BIMcloud / BIMcloud Basic to provide Teamwork. To verify this, run the Network Diagnostic Tool from the Open / Join dialog or the Teamwork Palette. Verify whether all necessary ports are open and the server machine is available from the client machine.
User Cache is corrupted
It is possible that current downloaded data have become corrupted, and thus are out of sync with BIMcloud / BIMcloud Basic. On the affected computer, try to leave all joined Teamwork projects.
Local Cache is corrupted
When more than one user is working from the same machine, leaving all joined Teamwork projects may be not enough. Delete the Local Data with the Local Data Manager.
Local Data deletion will require the user to rejoin the projects and all unsent changes will be lost. Try to leave all joined projects before attempting to delete their Local Data.