I was trying to sync DTProOffice to DTTG. I had just replicated a couple of folders into the sync group in one of my many separate databases. The sync started, got 5 documents into sync and stalled. Nothing was happening on Mac or iPhone. Clicking on the cancel in the sync popup window on the mac did nothing. After about 15 minutes I did a force quit on DT on the mac. When I went back in to open DT I got the error message about files being open but I opened anyway since I knew there was no other instance of DT open and running. All data in all 4 databases is gone. Even databases I do not have anything set to sync in.
A look at the console log indicated that my device was out of room. I assume that is what it was doing when I did the force quit.
My recovery was to go into Time Machine backups to a time when I was sure that the DT app was not open during the backup (to avoid any issues with corrupt files) and pull my older databases forward. I then opened them and did Tools Verify and Repair to make sure the data were all ok. I did have to re-do a few changes, mostly deleting a few items and some rearranging of folders but they were minor compared to the potential loss of everything.
I had already fixed it when I got the message from support to try this:
"Finally, please try this (for each database):
Select the database in the Finder, choose “Show Package Contents” in the contextual menu
Copy (!) the most recent Backup folder to the desktop
Copy (!) the Files.noindex folder into the copied (!) Backup folder
Add the file extension .dtBase2 to the Backup folder
Now you should be able to open the copied Backup folder by double-clicking on it. Afterwards use Tools > Verify & Repair (or even better, use Tools > Verify & Repair and Tools > Rebuild) to ensure that everything’s fine."
Which might be an option if someone else hits a similar problem.
While I am sure that my force quit was the initial cause of the data loss, I would have expected DT to report no more room and stop the sync rather than just hang there.