Unable to login to RT-AC68U's web gui admin after a few weeks of uptime?

I'm seeing a strange issue on my RT-AC68U's (one is primary, the second an AiMesh node, that is on a wired ethernet backhaul back to the main router). After a few weeks of uptime, I am unable to login to the webgui admin page (192.168.1.1) of the router, even though I am able to ping my router. Internet still works (that's how I am able to write this message) and client connections still works though. It's just that I can't bring up the web admin gui of the router.

A reboot or power cycle...

Unable to login to RT-AC68U's web gui admin after a few weeks of uptime?