Hi,
this sounds like a pretty severe issue that we are not aware of, there must have been a coincidence resulting in this issue
If this is really the case we are very sorry for the lost, do you have a backup of the files?
Should I understand that in your library you already had all the files from your DropBox location (local Dropbox folder) and then you went to Devices & Services > DropBox and on the 'Library content' tab you configured all 'Music' to sync without checking the checkbox [..] Delete other files and playlists... and pressed [Sync now] which wiped the files?
The idea I have (I will need to re-test myself to confirm) is that MediaMonkey wanted to sync/copy the library files to DropBox, but because the library files were actually the DropBox files then it somehow copied each file to itself which was problably the reason for the deletion
I'll test this and try to replicate and fix ASAP ( tracking as
https://www.ventismedia.com/mantis/view.php?id=20734 )
Thank you very much for reporting this and we are sorry for the troubles again :-/