Yes, that worked! I saved the document as IDML and opened it in InDesign. The text-boxes have been restored. I was able to delete them and save the document. The only thing I wonder is that the file is now much smaller than it was before (even before the cursed text-boxes appeared). It is now 45MB instead of 58MB.
Should I worry? Did I lose data (picture-quality)??
Anyway, thanks for the solution
Tami