I get an error message that the app can’t start because the database was modified by a newer version. I’m running 10.2.2619 so there is no newer version.
If you originally installed eM Client from the eM Client webpage, try then uninstalling eM Client and “dont delete the database” when asked on the uninstall wizard. Then download again V10.3.2619 from the release history page and reinstall and see if that fixes that error.
If that still does that same error, make a backup via “Menu / Backup”. You can see when the backup is complete in Show Operations via the dropdown on the right of Refresh.
Then if you happy to test a new upcoming release build V10.3.4341, go to the following page and download that newer version. Then close eM Client and install and see if that fixes it.