A Microsoft file hosting and synchronization service.
Solution:
"After two weeks and 52 reinstallations/installations of Windows 11 and 10, the solution to the problem was as follows:
(For the hundredth time) I disconnected OneDrive from my PC, went to the OneDrive web page, and deleted all the folders, putting them in the recycle bin. When OneDrive was empty (except for the "vault" folder, which is undeletable), I went into the recycle bin and restored all the folders I had deleted a few minutes earlier. I then returned to the PC, reconnected OneDrive, and at that point, synchronization resumed without any issues."