I see this on 4 devices. Those devices have been in production for 4 years.
It happens when the router http address is entered in the browser, right after the auto redirect to cgi-bin
It resolves automatically on reboot (nightly auto reboot solves this)
It happens again randomly, some times right away after first login, sometimes a few days later
No updates or configuration changes have been done for two years
I see this on RUT 955, 950, I don-t think I have seen this on RUT 240, could be a coincidence
Note that I do not access the router's web page frequently and the devices connected are accessible