Rule engine not yet started - not executing rule No idea why

Since several days I have a severe problem. Using OpenHab 3 on a Raspi 4 everthing worked fine over months. Suddenly no rules were executed. The trigger is triggered (confirmed in the log) but for every rule the above mentioned error message is sent. I installed the system twice, restored the backup of my installation and struggled into the same problem. All rules are text files in the directory.
Any ideas? Thanks a lot in advance!

Chaning the SD card did not change anything. After reinstalling the BackUp the same error. So I decided to restart from scrap without using any BackUP. I only used item, persistence and rule files and created all Things new. That worked and solved the problem.

So after a couple of hours running without problems I do now have the same issue.
What for heavens sake causes that error?

Still the same problem. I changed to a Raspi with 8GB but no other behaviour after reinstalling from backup.

Today I resetet everything again. New installation and new set up of the whole System “by hand” that means persistence and items are imported with files, all bindings are installed by hand, rules just copied into the directory. It worked fine at the end of the day. So I decided to do a BackUp. Stoped the system cloned the SD card. After restart same error messages. Interestingly also the log4j2 file is changed. Frontail only let me know which errors happened (none). I had to change the file to “Debug” in all Openhab items to see the same story as before. By the way, this was the very first hint to this desaster weeks ago, then Frontail suddenly seemd to stop working but in fact only changed from “INFO” to “ERROR” mode.
I do not know what to change and how to solve this problem. If anyone has ideas, he is heartly wellcome.