I use DT on my Mac, my iPhone and my iPad. Now I run into the problem “Duplicate database UUID” when trying to synch. What to do? I can’t really describe what I have done before, what could have created the problem.
Did you duplicate a database in the Finder? Or open a database and a backup of the database at the same time? These are the most common reasons.
I am not aware of that but I have to check. I had problems with my (external) SSDs. So I had to open an older Version from Time Machine. Maybe that is the problem. Would it help to create new DBs and move the files to these?
Did you open multiple versions of the same database at the same time? Only switching back to an older version shouldn’t cause this error.
This would be the case as I normally do not close DT when I finish working on it. That is the case for Mac, iPhone and iPad. My plan would be to focus the synchronisation on one DB at a time to find out which DB creates the problem. Let us give it the name P (for problem). Then I would create a new DB S (for solution) (let’s say on the iPhone) to which I would move the files from P and synchronise that one (S) with the Mac. So that I have S also on my Mac. Then I would merge P and S on the Mac and delete duplicates. Then I should have all files from P as well as the additions in the new S. Would that be a feasible approach?
The easiest solution is probably to replace the current database in the Finder with the desired older version from Time Machine while DEVONthink is not running. But first I would recommend to back up or compress the current database.
I thought I did exactly that, but I may be wrong. As the content was and is quite critical I was a little bit excited to say the least 
That’s exactly why creating backups (the more the better) is usually our first recommendation.
Absolutely correct. But when you see two Samsung SSDs with originals on the one and backups on the other failing more or less at the same time → Houston, we have got a problem! Disk Drill offered me over 180.000 files with let’s say only limited value because of the sheer number of files. In the meantime I was able to work with a Time Machine backup from another drive accepting to have lost some work of a couple of weeks.
What exactly did actually fail?
The first was no longer mountable. However you could see it in Disk Utility. The fixes there did not help. For this reason I used Disk Drill and stored the results (the 180.000 files) on the other SSD. But before I was able to really go through all of these files the second one was suddenly no longer visible also not in Disk Utility. This has not helped to increase my confidence into the Samsung SSDs. But, shit happens and life sucks. So now I work with what I have and care for a comfortable work environment a little later.
We recommend as much backups (ideally on multiple devices and locations, e.g. in the cloud too) as possible and especially not to rely only on Time Machine.
Anyway, are you able to open the older, restored version? Is a verification via File > Verify & Repair Database… successful?
Yes I was able to open earlier versions. But as I had changed the DB structure of my setup in the meantime, some work is lost. So, for the time being I will focus on my Mac and work on the synchronisation later. (It also helps to reduce complexity and thereby increases my focus;-))