iCloud iCloud synchronisation issues

[icymountain]

I have been getting plenty of synchronisation issues with iCloud for a couple of days:

• Files not synching at all (either from device to iCloud or the other way around), both on MacOS and iPadOS;
• Very high cpu usage by fileproviderd, bird, and cloudd on MacOS;
• Finder says there is a very small amount of data to synchronise and stays like this forever (I have seen it show "1/2 bytes" for 30 minutes);
• Sometimes the Finder gauge of files to synchronise also goes up without any file changing in the background
• File conflicts with applications that synchronise via the cloud (I had to fix by hand about a hundred duplicates).

It seems each of my devices fails to synch, sometimes resumes, but mostly does not work anymore when I expect it to. I sometimes had temporary iCloud synchronisation issues in the past, that would resolve by themselves after a while. But this time does not look quite the same.

I am not sure where to start to troubleshoot this ?