I spent several hours to fix this problem. My phone call (1 hour) at Apple Support did not fix the issue.
Now my problem is fixed! The workflow:
1. System preferences “Monitors” set to 32768 colors
2. FCP started (still the same problem)
3. FCP terminated
4. System preferences “Monitors” set to 16,7 Mill colors
5. FCP started – the problem seems to be fixed
Best regards
Chris