It seems ultramixer doesn’t always accept files to be added to the playlist.
I could provide the umerr.txt and umlog.txt where the last action was adding the file concerned. Pls tell me how to provide the files to you.
In addition I recognized that the playlist sometimes get’s deleted on various occasions (not only when selecting “delete all files…” 
It’s a known issue and fixed in UltraMixer 2.0.3.
(postet several times in the old forum at ultramixer.com/index.php?c=Zm9ydW1vbGQ=)
Files are removed from the playlist after playing (depending on your preferences). Otherwise it’s a bug and we have to check this…
Best regards,
it seems to be a bug. The playlist is deleted completely on the player side where the file should be added (no relation to specific files).
I know about the behaviour that the files played completely get deleted afterwards.
You’re right! It’s a wrong behaviour - we will fix this issue in 2.0.4.