thanks for your input.
I started by factory resetting my RUT240. Then I upgraded (2 step) to the latest RUTOS version RUT2_R_00.07.03.4.
I then proceeded to configure the router and noticed that I was unable to change the WebUI settings.
I finally edited the /etc/configure/uhttpd file to make the necessary changes, incl. my self signed server certificate which where also being ignored by the WebUI.
Most of my changes now show up in the WebUI setting page except the certificate and key file names. These fields still show the default values.
I'm still unable to make changes through the WebUI. I will look again in the system logs to see if I can cache any error messages.
Thanks for pointing be to the UCI, I will give it a try.