If the file is corrupted try to export it as IDML, open it and work with it.
If this does not help, try to open IDML in a XML editor (there are several like BBedit which are able to read inside zip files) and try to find the corrupted element and to repair it. Do it on a copy, because a failure might destroy everything.
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
IDML is a XML, zip compressed file without previews, that is why it is so small.
But also a new opened file ans saved as INDD can be much smaller. Often you will find unused code or at least code you will never use again. The code of such a file is build up new and more efficient. This will cause a reduced file size. Specially documents converted from older versions, or which have a conversion history from Quark Xpress or PageMaker will loose a lot of "weight". I had cases when the file size was reduced to 10% from previous versions, but also where only a little bit insignificant amount changed.
Don't worry about the smaller size, check only, if all styles have not changed. Sometimes automatic numbers or bullets get lost or similar things happen.
Ah, thanks for the info, I appreciate that!