One of the few people apart from Apple who seems to know much about iCloud is Howard Oakley. His website has many pages dedicated to the oddities and problems it throws up. It is well worth keeping up with what he writes. Here are a couple of links:
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PS: those using Sonoma might like to read this, which incidentally has an explanation of what CloudKit does:
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After fighting a number of different issues over the course of years, Iâve found the most reliable way to sync is to stand up a WebDAV server on my own personal NAS and sync every device to that. It works every time. I use a Synology NAS, so their software largely takes care of the certificate and DNS issues for me too, so I can sync remotely from my iPhone, iPad, etc. while Iâm out and about.
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@bbrip it shouldnât matter, but who knows with Apple: whereabouts in the world are you, and is that likely to affect connecting to American servers?
Also, are you running a VPN?
Even at its worst Iâve not had the CloudKit error not resolve itself after maybe half an hour, and the problem is intermittent. If youâre getting the error every time and itâs not resolving, there would seem to be a different problem.
I was able to solve the same sync issue by completely recreating the DevonThink sync settings on ALL devices. Since then it has been running smoothly for months.
Unclick everything, close DevonThink, reopen it and recreate all sync settings.
That was the way for me.
BTW: I disagree with the others that the Apple cloud is so unreliable.
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No VPN. Germany. Never had any issues with iCloud with numerous other non-Apple Apps that sync to it.
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Already done that numerous times. Deleted old one, recreated new CLoudKit sync settings. To no avail.
Me too.
Did you just delete the sync location or actually clean it first?
Only deleted it, how do I âcleanâ it?
Sync locations can be verified/cleaned via the contextual/action menus in Preferences > Sync.
ok, did the same exercise with cleaning the sync, then deleting it. Once I set it up anew, it starts uploading but then in the end I get âError 15â again. I give up on this.
How much iCloud space is actually available and how large is the database (see File > Database Properties)?
I am still in test-phase so my databases are tiny. Alltogether < 1GB.
I got a ton of iCloud storage, At least 230GB free.
I have something syncing error-free now using iCloud Legacy. A start, I guess 
People are free to have their opinions, but we have a large volume of data that counters that opinion. Thatâs not only from our own support channels but from outside them as well.
And as weâve also discussed often, itâsâŚ
- Not something that effects every one. However, the number of people it affects isnât insignificant.
- Not something under our control. We donât control Appleâs servers or the intervening networks between them and anyone.
- Not something we can detect or anticipate could happen to a particular person. If we could, obviously we would.
And again, we do not recommend using a remote sync option, unless one is actually needed. Just having an iCloud account does not make it the optimal choice for a sync location. We make recommendations to try to facilitate the more robust and pleasant experiences. And in the event someone is in the situation of having a remote sync issue, itâs incumbent on us to make such recommendations.
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using iCloud Legacy. A start, I guess
Legacy doesnt mean deprecated or abandoned. The mechanism isnât the same as CloudKitâs and iCloudâs underlying processes are handling their own uploads and downloads. This means we also donât control that, so you may have to be patient with a Legacy sync.
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A screenshot of File > Database Properties would be great, this would help to estimate the approx. amount of CloudKit requests.
Thanks for the screenshots. However, the amount of data is actually quite limited. I just uploaded a database with 10 times more data and 100 times more items (meaning a lot more CloudKit requests) without any issues on Sonoma. Weird.
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As I said, still a testbed and very small. I am ok for now with iCloud Legacy.