After proceeding with "Restore to factory defaults" the router should reset itself and no signs of configuration should be left.
Yes, the latest Firmware RUT955 is installed. No custom reset button times were changed. I just used the Software Button on the Web GUI and afterwards I even disconnected the power adapter for a few minutes.
Very strange - with RUTX11 I did not have such experience...
Next time I will try the physical reset button.