I had the same problem with iCloudDrive on Windows 11 for a long time. Nothing synchronized. All the “quick fixes” from the forums with “Reset iCloud for Windows” etc. never helped me. Anyway, today I found out in the depths of the internet that the email addresses associated with the iCloud account are suddenly (- for whatever reason??...) Case Sensitive...
So the solution that worked for me: Reset iCloud for Windows, log in again (but with the email address case-sensitive - just like you see it in the iPhone settings) and now everything works!
(Specifically, I always capitalized the first letters of my first and last name in the email. In the iPhone, however, my entire email is in lower case... And that was probably the problem. To my knowledge, however, emails are not otherwise case sensitive. So it's very strange what happened.)