Not really - it’s a plain, fresh openHABian installation, two days old. And I did nothing with REST API - at least not knowingly. I have no idea where this comes from. Do you have a hint where I can look at to get to the root cause of this warning?
The thread title said openHAB though. They are 2 different systems even though openHABian uses openHAB. That is like calling openHABian Raspi OS because the openHABian image uses that OS.
I think the error occurs when the web interface is already accessed during the startup process.
Close the browser and restart openhab. Wait until the startup process is safely completed and then check if you got a warning.