Can't access management login page

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I have two systems, TVS-471 and TS-451, both running 4.2.4 (20170313) firmware with static IPs on my home network. Up until yesterday afternoon, I was able to access both machines either locally or remotely. As of this morning, I can access the TVS-471 either way, but I cannot access the TS-451 at all. Pinging it receives the normal response, and the website that I'm hosting on it is still accessible and fully functional, so I know the NAS isn't frozen. When trying to access it remotely, I get a Host Not Found error. Locally, I get "This site can't be reached". In both cases, it seems the connection is being refused. On a local PC, when I run Qfinder Pro, both devices are seen and I'm able to access network drives, access the configuration and get details, but when I try to login, upload media or access the Resource Monitor, I get "Login failure: Invalid username or password" (although the same credentials work fine for the other functions). Unfortunately, I don't have SSH enabled on this box, so that's not an option. Also, I can FTP into it. It's almost as if it's refusing connections on 8080 (since port 80 is passing along to the web server just fine).

[spepin]

Update -- I was able to perform a reboot from within Qfinder, and after the NAS came back up, everything is working again. Still trying to understand why part of the NAs was fully functional (web server, file access, etc.), but the management interface and some other features were dead.