I think I have found the culprit. It seems to be the iOS app for accessing OH.
I started the app and suddenly got the “Jetty warning” whenever I did something in the app.
After restarting the app it went away and I have not been able to recreate the error.
But I found this thread: IOS App issues - no connectivity, stability issues
So probably this is what causes the problems.
I’ll leave it for now and continue to monitor the thread above.
Thanks for your fantastic and rapid support!