This is a rephrase/clarification of a post I made a few weeks ago where I only really figured out the bug way down in the comments after a lot of wrong turns.
I'm using Library Server at home and connecting to it from work. At home all of my files are on a server called, cleverly, "files". So the "Filename" property for tracks stored in my library look like this...
\\Files\Music\The Beatles\Let It Be\001.Two of Us.flacI installed Fiddler on my work machine to watch the outgoing http requests. When I start playing a track I see a request go out for
http://files/Music/The%20Beatles/Let%20It%20Be/001.Two%20of%20Us.flac... which stalls for about 10 seconds and then 502s. Of course. That's the local UNC path that would work at my home "turned into" a URL and tried on a different network. Slightly spooky to think I'm hitting whatever random server at work might be named "files" (if there even is one). Anyway, as soon as the bogus URL times out, MC does the right thing and requests ...
http://www.mydomain.net:1234/^5C^5CFiles^5CMusic^5CThe^20Beatles^5CLet^20It^20Be^5C001.Two^20of^20Us.flac?conversion=3which nicely works.
Interestingly, the bogus get doesn't happen on every play. If I start a track, listen to 5 seconds, then another, then another, there's only one request per song, the correct one. Seems as if waiting for a while and maybe jumping between albums / navigating "far away" in the library is part of what provokes the evil request.
A few days ago I had the "play local file if one is found that matches" checked (suspicious!) but I unchecked it and the problem remained (even with, eventually, reboots of every machine involved).
At the moment I've added a bogus "0.0.0.0 files" line into my hosts file at work so that the bogus URL will 502 instantly. Works to alleviate the symptom. But the bogus requests are still happening.