I tracked the issue down by myself:
- In the FAQ I found the entry how to reset all QtCreator settings. After doing this the QtCreator worked again.
- I wrote I didn't want to reinstall the QtCreator (because of the settings), so I restored the old config from the bin and tried deleting the corrupted config file only.
- When I finally deleted
QtProject/qtcreator/mimetypes/modifiedmimetypes.xml
QtCreator worked again. I remember messing with the mimetypes prior to the crash, so I guess that's it.