Dear rlkoshak,
thank you for your help.
After some additional fixing and some reboots i am now getting no warning anymore.
It was some work but now i think everything is running stable on openhab 4 and the RPI 3b+ 
Dear rlkoshak,
thank you for your help.
After some additional fixing and some reboots i am now getting no warning anymore.
It was some work but now i think everything is running stable on openhab 4 and the RPI 3b+