After updating to Version 10.5.7.0 DOpus worked without reboot. After a restart it doesn't start any more. Error message: "The English Language Pack could not be found!" Thank you for your help.
Please try running the installer again and rebooting at the end. It sounds like files set to be replaced on reboot were not for some reason.
I tried but the error message remained. I then uninstalled DOpus completely, rebooted, installed the latest version, rebooted again. It works now. Still a bit strange.