Ok, all stories have an ending..
All of a sudden I remembered that during preparation, the application crashed just after importing the last quicktime. When I restarted the application the quicktimes were gone. (at least in the bin they were.) So, not willing to wait for more then an hour again while importing quicktimes, i opened the mediatool and found them there. I just dragged them to my rushes bin and I could start the edit. I felt clever.
There was my mistake.
These quicktimes had gotten, in hidden way, corrupted by the crash. They couldn’t be modified because of that. When I imported them again (and that means importing them as new clips and not batch-importing the old ones) they could be modified just fine.
So I recut my old edit with the new imported quicktimes and I was troublefree finally.
thanks for all of your help.
Martin