@Hudson555x: Sorry and thank you, I didn’t see the popped out Playlist window! Popping it back in place solved the layout issue.
@shotcut/Dan: The scaling problem appeared right after the update from a quite old release. I don’t recall the previous version number but guess it was about 2 years old. I did not change any display settings, otherwise other programs would have probably also been scaled up, which was not the case.
However, because of your remark, I tried to activate compatibility settings just for Shotcut and that solved the issue. Font-size is back to normal now (see screenshot) on both the internal 4K laptop display as well as on my external Full-HD monitor.
Maybe I activated compatibility-settings back then for the previous version as well and this setting now got lost due to the update.
Thanks again!