Update: I performed a clean install of High Sierra. Up to now, the problem seems gone. I've exported (to file) 7 videos: 2 with clips coming from only one video, and 5 with clips coming from 2 different videos.
However, compared to my previous attempts when High Sierra was installed as an upgrade of Sierra, also the following things changed:
- I didn't test yet the files that gave me error in my "upgrade-installation" (sorry, I didn't have time).
- The menu bar is always shown. In the previous attempt the manu bar on the top was set to auto-hiding.
- I'm exporting keeping the lid of my MacBook open, otherwise the creation process become very slow and the machine starts to lag while opening folders, app, etc.
Someone could test the last two points of my list, please? If nothing work, I will try to export the files that gave me error in my previous installation, but I will do it during the week-end. During the week-end I will also try to export videos with clips coming from more than 2 files.
Anyway, I will keep you informed.