Since my original message began "I get this message when I try to use iCloud on Windows 10,...", the Windows 11 advice is not applicable.
I've already tried every reasonable suggestion I have seen on the Microsoft and Apple support sites. It is unreasonable, for example, to suggest that I backup my computer and reinstall everything simply to access iCloud. If I have to live without iCloud drive, I will simply revert to emailing files to myself to transfer them. At least ICloud email still works.
I never asked for this. I was satisfied using the version of iCloud that I already had when this message started appearing. As far as I can tell, Apple provides no way for me to return to the version I was previously using.
Does anyone know what Error 101 means? That was not listed on either support site.