iCloud syncing just stopped working the other day. I’m getting the message in the topic title. I saw some older messages with similar problems but they didn’t seem applicable. I’m on DTTG 2.7.4.
Is this also syncing with a Mac on macOS Catalina?
Is this just iOS 13?
The sync on Catalina is working and the iOS 13 sync started working after a reboot. Apologies, should have tried that before posting.
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No worries and thanks for the follow-up. 
iCloud has been a mess (and not just for DEVONthink) in Catalina and iOS 13, with some fundamental changes they were working on through several betas, then suddenly backed out near the end.
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I have the issue now.
I am trying the trial version of DT3 on 10.15.1 and created a new database just for RSS feeds. When I switch to DTG (2.7.4 on 13.2.2), I get the aforementioned notice and no database is created. I sync over iCloud.
We assume that this is an internal problem created by the back-and-forth of iCloud changes in iOS 13 since the first public beta. As a remedy, and because on iOS we cannot just cd into the iCloud directory, I’d suggest to deinstall DEVONthink To Go, reinstall it, and resync. This should delete all the obscure and maybe faulty directories that the iCloud experiments of the recent months have left behind.
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I did restart. I will reinstall then. Hopefully, I will get at least my original database back reinstalling.
When I reinstalled DTG, the sync settings told me over and over that my encryption key does not match. I use 1Password to manage that and it I doubt they were different. I had to clean iCloud removing the sync. I did not manage to make a new one with an encryption key (the same message).
However, it finally work without encryption. Which is good. And bad.
Are you using dark mode in iOS?
Yes. It is permanently turned on. Is it a problem? And since we are at it, is a dark mode coming to DTG? 
Switch to light mode and you would see the two required fields in the sync setup in DEVONthink To Go.
Dark mode is planned for a future update.
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Ohhhhh, so the problem was that I did not confirm the key! Okay, I will turn on encryption now. Thanks!
You’re welcome. Do note, you can’t just add an encryption key if you’ve already synced. You’d need to clean the sync location first, then enter the key and sync again.
From Help > Documentation > Troubleshooting > Sync > Common Questions…
I was getting this error on first one, then the other of my iOS devices. Rebooting them resolved the issue in both cases.
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Thanks for the confirmation.