Please search the forum for that error messages and you’ll see that for some people (many who report here) how this is a message from Apple’s servers. For some, Apple’s servers have been unreliable for months. You might try Apple CloudKit, if you really need a third party internet synch service. Otherwise, also or instead of, try Bonjour. More reliable and quicker.
I can do without iCloud for now, but probably not in the long run. I disabled it for now. Bonjour is enabled on the Macbook. But on iPad the cloud button is completely greyed out (see screen print above), so no idea how I can activate bonjour there. Any clues would be appreciated…
There is no solution for Apple’s network errors outside of Apple. As many of these threads have explained. Your best bet might be to use Bonjour or another sync method. And these are detailed in the manual as well as here, as well.
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Simplistic, I’d say. DT announces that you can sync using different services. They do not provide these services. You decide to use an unreliable one – that’s not the fault of DT. You could, for example, go for Bonjour or Dropbox or WebDAV. I am, for example, using WebDAV, and I never see this kind of problem (nor any other sync trouble). 
It’s like buying a fridge and complaining to the company that built it when your electricity supply breaks down. Or complaining to DT if you can’t print something because you’re out of toner.
DT does not promise that CloudKit works flawlessly. It just says that if CloudKit works flawlessly, you can use it to sync your databases.
 Well, occasionally my Synology box decides to go to lala land and not accept network traffic any more. Not DT’s fault, though.
Whilst this comment is not directed at OP specifically, I am getting a bit fed up of people (not just on this forum) defending iCloud by saying “well it works for me”. It’s nice that it works for some people (as far as they’re aware - you don’t actually know it isn’t working until you try to find something and it’s missing….), but that doesn’t erase the varied experiences of thousands of other customers and the widely documented problems with iCloud, which Apple itself occasionally acknowledges when it fees like it. It’s not even just third party apps - there are widely reported issues with native Apple apps.
Yes, we all* want iCloud to work well. Apparently that isn’t a priority for Apple, but don’t diminish other people’s frustrations just because you’ve not experienced problems yet.
*well, most of us.
For what it’s worth, I am running the iCloud sync between Mac, iPad and iPhone, and I just ignore the error messages. I know it’s Apple being annoying, and errors usually resolve after a few minutes.