I´ve carefully followed the " Hassle-free openHAB Setup" step by step https://www.openhab.org/docs/installation/openhabian.html. I´ve added a few things like TV, lights etc., created some rules and everything was fine.
Probably something went wrong when I added the DHT22. I´m not very familiar with linux and I couldn´t find an easy to follow step by step how-to. So I had to do a lot of testing, finally I did it this way: https://community.openhab.org/t/how-to-show-dht22-stats-on-hab-panel/39853/16. And then the warning occurred for the first time.
Anyway, I will set up a clean installation and report if the error continues to occur.