thank you very much for this hint.
It is a little bit strange: This solution works (the clock-app starts after reinstalling) - but after a phone-restart the issue appears again.
For me, the circumstance is not too tragic, since I planned to reinstall the system anyway.
If I could have solved the problem, I would have postponed the reinstallation to a later time.
Now I wait until the official version is available and then do a “Sailfish restart”!
(There are some other little problems that I hope to solve this way).
Thanks for the feedback.
Edit: Now I have it! After I read the original post then but correctly. First “pkcon remove harbour-dynclock”, then again “devel-su pkcon install --allow-reinstall jolla-clock”! Now it works even after a reboot.