Media Player Meltdown
I'm glad I am not the only one. I regularly use Media Player for managing my workspace home audio; I have some add-ins to manage FLAC files, but (with a few finicky set-up rules) it mostly works seamlessly.
I had a number of album updates to compile over the weekend, and the program cannot look anything up now (partly because in my case, I have to use the type-in album lookup interface). While I had worried/presumed it was something on my end (I use Windows 8, but did recently add the "classic shell"), an "Internal Server Error 500" typically means it is not my end.
Like you, I could sometimes get to the lookup page, but then it would default to the "Internal Server Error" response when I tried to select a library-album. I've started to consider other options, such as AudioMonkey. I rip using dBpoweramp. I do like WMP as it is part of the operating system and like things to be as smooth as possible.
I had to post as yours is the first post I've seen, anywhere, and this problem has been going on at least since September 9th, 2014.