Volumio very slow at playing songs from NAS

Sorry for cross-posting, I originally posted this to “Help and Support”, but it might be more of a bug report. This is regarding Volumio 2.917 on a Raspberry Pi 4.

My setup is the following: I have two instances of Volumio (“upstairs” and “downstairs”) and my music library is on an external hard drive attached to “upstairs” via USB. This gets automatically shared over the network and in particular acts as a NAS for “downstairs”. This works fine, except for playlists. I have quite a few long (several 100s of items) playlists that I copied over from “upstairs” to “downstairs” and edited so that the URI’s point to the correct location. Whenever I try to play one of them though, the GUI freezes for a very long time (about half an hour!) before playback starts. During that time, each song generates two entries to the log of the form

info: Adding Item to queue: music-library/NAS/Upstairs/Music/Path/To/Song
info: Exploding uri: music-library/NAS/Upstairs/Music/Path/To/Song

followed by

info: sendMpdCommand stop took 149266 milliseconds

The problem is that the ‘exploding’ part takes about 6-7 seconds for each song, which quickly adds up to insane amounts of time! The same happens when I simply play one album, but then the total time is “only” about a minute or so.

Interestingly, this does not appear to always happen, but it does most of the time. I haven’t figured out yet if I can make this more reproducible, but am happy to experiment. Another thing to note is that I sometimes get the message

Upnp client error: Error: This socket has been ended by the other party

(after a rather long timeout) when trying to start playback. As a result, my current setup is unfortunately pretty much unusable.

Am I doing something wrong or is this a bug in Volumio?

Edit: A log can be found at this link. I also just noticed that in Settings → Sources, “upstairs” shows as “not mounted” (red cross in the “Mounted” column).

Edit 2: The problem does seem to be caused by the NAS not being mounted properly. Right now it shows as being mounted and everything works perfectly. The mystery then (to me) is why it sometimes gets unmounted (the “upstairs” instance of Volumio was up and running all the time and both are connected by Ethernet) and how it can be mounted again when this happens. A related question is why playback works at all when the NAS is not mounted? Is there some sort of fallback mechanism that works, but very slowly, and if yes can this be disabled?