i broke down my Project.
Deleted all unneeded Tracks (only 2 left) - bug still there.
Deleted all other clips - bug not there again.
So i now have a Project with 2 tracks, 9 clips and 9 fadings.
Then i tried to remove all other clips from the filelist exept that two - bug not there any more.
Then i removed all clips exept the 9 used clips, and yes - Bug still there.
I copied the clip with that problem and the bug is there too.
I removed clip by clip, and in the end there are 3 clips and some fadings