Installed Gizmo, and tested if the issues remains: There is no issue when using Gizmo, each and every track is displayed and can be played!
2) "Is it always the same albums which exhibit the issue?"
Correct it is always the same subset on albums, I believe. When I add new albums, some are affected others not.
4) ... "Does the server remain visible on the network when you encounter this issue (look at available zones in JRemote2 for instance, or another app)?"
Sorry, no idea how I could check this. Any advice pls?
Your extensive testing has eliminated a lot of possible gremlins. Further suggestions:
Re 2): If it is truly repeatable, leverage that by picking one specific album and take a really close look at its file format, bit rate, and especially the tags. My impression from earlier posts is that you probably are dealing with a lot of classical albums, which can be very tricky to tag:
A) Do your tags have special characters in them (umlauts, accents, non-English letters, etc.), which may mislead JRemote2 whereas Gizmo is more robust in this regard? If so, try removing them and see if that fixes things.
B) It is a known issue that invisible characters sometimes creep into MC tags, particularly album names. I have been frustrated by this multiple times. Hard to detect. A comparison "cross check" smartlist can be revealing if in your organization the album name and folder name are supposed to be identical, but turn out not so. If suspected, I totally delete the album name (or other questionable tag) and re-enter it manually. More often than not, voila, the issue vanishes.
C) Is your audio file structure relatively straightforward (for example Artist > Album > Tracks), or more complex as may be the case for classical music? Are you mixing classical and "non-classical" which have different important tags filled out or empty, and your exceptions always fall into just one of these categories? I think some users have a dedicated file structure for classical only.
Re 4): Since you report that Gizmo works, I think you have eliminated most server issues. Some long shots:
D) You can check if the server is "lost", i.e. not communicating with JRemote2 only, or entirely down, by firing up Gizmo on your phone and looking at its server list when JRemote2 is spinning its wheel. Even try shutting down JRemote2 and immediately try to play same album on Gizmo.
E) If JRemote2 <--> MC Server network communications are suspect, play around with settings in "DLNA Controller Options" (RMB menu for your target device in Playing Now). At the moment, I have the last three out of the four checked for my particular system. If you have multiple player zones, does same issue arise in every zone?
F) If you initiate playing a troublesome album directly on the server, does JRemote2 correctly "monitor" the tracklist and currently playing track under its Player "Player" zone? Normally this should happen.