OK. Now I found a solution by my own.
I did the following: (just if somebody gets someday the same issue)
- Stopped openhab sevice
- openhab-cli clean-cache
- I edited /var/lib/openhab/automation_rules.json an removed everything except the {}. Everything I tried during the last days was there, as well al some old rules I made with OH 2.5 and its Next-Gen Rule Engine.
- Restarted Openhab
Voila, it works, now I am able to create rules in OH3 as expected.
Seems that this is / was a migration issue in connection with the NGRE of OH2.5
Greetz