[sciurius]

I played a bit with synch to cloud and to backup, but these do not seem to be useful yet, is that correct?

For example: synch to an empty drive folder seems okay, a subsequent synch (with no changes inbetween) is painfully slow (approx. 10 songs per minute, gradually slowing down to less). It merges quite a lot of songs (again: there were no changes at all).
Also, it is not clear what the 'client' and 'server' roles are in this case.
EDIT: Killed the synch after 5 hours and only 21% progress.

Synch to a backup file quickly leads to a java crash.

Unfortunately, in contrast to synch to device, these functions do not write a log file so it is hard to report details.