why would my OneDrive suddenly be processing over 40,000 changes?

It might be worth changing the setting in Windows to identify your mobile phone as a "metered connection"; this generally causes OneDrive to pause any activity. Funny you should mention Nord, I also use that. Seems weird that might have this impact. Since posting, I've gone back up to 170k files!

Same issue... I now understand why my mobile phone internet data vanished some time ago after using it as wifi access point for my laptop: This undesired synching has sucked dozens of Gb and is repeated without any control.

What I still don't understand is why I see this... I just have the feeling that in my case it happened after using NordVPN which somehow makes my W11 OS PC very uncomfortable when activated (every action taken on PC goes very very slow and shutting down completely NordVPN services kind of solves the problem).

Could it be related to NordVPN or VPN in general?

Yesterday I noticed that One Drive would not sync. Word and excel were returning a message along the lines of "One Drive Host is in read only state, changes will not be synced at this time... try later...." Wish I had screen shotted that. Now today, right after first login, all my files are syncing. No message from Microsoft about the "read only state", that's a bit disconcerting.

Hi guys, I have a helpful reply!

Due to orders from a particular organization, we are doing something to your files and metadata associated with them to stay in compliance.

Please do not worry, this is for your safety and covered under the most recent EULA to which you agreed (including compliance on this discussion forum.)

/sarcasm

I have exactly the same problem as you mentioned under 1.