Transitions don't work

I have a problem with the transitions between two clips.
The clips are from the middle of longer video snippets, so there are still many seconds/frames before and after the part you see in the timeline. If I e.g. "CrossDisolve" the area is marked accordingly, but it remains a hard cut. When I walk through this area frame by frame, I don't see any change until the cut. Only when I drag the first clip into another track, lengthen both clips accordingly and fade the upper clip down over the area, the desired transition occurs.

I bought Studio now and did not have such problems with the free version 14. I completely uninstalled the 14 before I installed the 15.

does you system meet the minimum requirements? Check the release notes. If your system isn't supported then you are out of luck. And bug reports are no good without full system specs.

Well, a quad-core Intel CPU, 12GB RAM, 4GB AMD CPU with OpenCL1.2 and SSD for the system and data.

I can play in fusion with Nodes and do great things. I usually work in 1080P and rarely have more than 2 video tracks in use. I prepare audio in AdobeAudition beforehand, so only one stereo track or two.

That's all possible. Slowly, of course, but without problems. Only a simple crossfade is not possible. For example, I specify that I want to do +/- 24 frames crossfade and this will also be displayed in the timeline. But there's no Fade. Neither in the preview, nor in the finished film.
Now I have to put the clips alternately on two tracks and fade in and out the top track. Then I have my crossfade. And with the 14 it went without problems.

What clips come before and after?

The scenes (10-20 seconds) are from the middle of longer clips (some minutes) . So in the transition-area are some minutes of video spare.
All Clips are 1080P60. But the Indicator for the transition stays red, if I try to make the transition longer or shorter.

But on two separate tracks it's easy to overlap the two scenes and improvise a crossfade there by fading the upper track. So this prooves that there is enough footage for this purpose.

Here is the improvised video (not finished yet)

Some Ideas? anyone?