kardac

I solved it.

1. Changed theme to TwentyTwenty
2. Disabled all OceanWP extra plugins (noticed that I had the old OCeanWP Floating Header plugin which havent been updated in a while and is not an official plugin from the makers of OceanWP.
3. Activated Really Simple SSL
4. Switched back to OceanWP
Now i can enter the CSS settings. I manually edited all http URLs to https.

Everything works as a charm.
I suspect the problem was OceanWP Header Plugin, but i cannot be sure and I dont have the spare time to reload a backup and test 🙂

Thank you for all replys!