I have seen many posts on here talking about admin credentials disappearing, and the password going back to by itself. In every instance, a reboot has fixed the problem, but it always leaves the router wide-open to everybody until rebooted.
Here are some of the threads detailing the issues:
We don’t have any reports of this in Support. If you have such problems, email support with your supout.rif file that you make after the problem appears
I took camtasia video of this happening, as well as supouts before and after. The supout during this event never finished because the router is mostly unresponsive to CLI tasks. If I see this again I will make sure to take more details, as well as send to support the info.
I run about 20 different MT routers, and this only happens on a single router. It has happened on 2.9.x as well as up to 3.16 that I know of. Seems about 3-6 weeks intervals is when it happens.