Thanks for this mega-quick reply! The link to the text "writing a good bug report" leads to a text which has to be written.
But: The tip to clear the cache led me to my profile - and I discovered in ...\appdata\local several folders for RawTherapee. I uninstalled RT, deleted the profile folders, reinstalled the latest GTK3-Release - and everything worked again.
So: Thanks again - your help saved my work!