Davy, I'm 99% there in getting UAPP working in the mobile network by tethering MinimServer from BubbleUPnP to UAPP.
BubbleUPnP Server just received an update that lets you transcode >96kHz FLAC to 96kHz FLAC (good for streaming bandwidth requirements). However, UAPP reports the "Duration" metadata for all these transcoded files as "-596523:-14:-8", which makes the UAPP playback progress bar (or "dot") permanently stuck at the end and no song length is shown. Otherwise playback and controls work fine, i.e. the issue is visual. Meanwhile, the BubbleUPnP UI reports "Duration" metadata correctly.
For all "native" FLAC files (i.e. files not transcoded by BubbleUPnP Server), UAPP shows "Duration" metadata as "00:00:00". But this is not a problem because the playback progress bar works and correct song length gets displayed in UAPP's "now playing view".
EDIT: I originally wrote it's a mobile-only issue but it also happens in LAN. Though Im sure I saw the correct "Duration" metadata on at least one occasion in UAPP when in LAN. Sadly, I can't say what the logic behind this behavior is.
Does this sound like a problem with UAPP or the UPnP tetherer? Like I said, it doesn't totally break playback but of course it would nice to be able to see the progress and overall length of the playing song. Anyway, I thought it would be worthy of reporting.