If you try to upload or change an event, task or contact but an error message pops up with the reason described in the message (e.g. invalid value, missing or empty field) it means that there is an invalid item that you're trying to synchronize with the server.

Try to delete the local changes to stop the synchronization – if this cannot be done manually or the error message cannot be closed (or keeps reappearing) it might be best to clear the local cache memory in eM Client for this account and download the correct data from the server.

Simply right-click the affected folder (calendar, contacts folder, etc.), select Properties and open the Repair tab. Click the ‘Repair’ button and wait for the resync to finish.