Ok as disabling the Security program and VPN made no difference and you use no encryption, then check your “Netframeworks are up to date” via the Windows updates.
If Windows is all up to date , suggest next to try uninstalling eM Client and “don’t delete the database” when asked on the uninstall wizard and then reinstall V10.3.2619 via the release history page and see if that fixes it.
Next If uninstalling and reinstalling makes no difference try running a database repair as per @Gary post in the following thread.
Note:- Before running the repair, make a manual backup via “Menu / Backup” incase you need to restore for any reason. You can see when the backup is complete in Show Operations via clicking the drop-down on the right of Refresh.
If then running the repair makes no difference then suggest next if you have a paid active Pro, Personal or Business version, go to the VIP support page and login and lodge a support ticket.