A Microsoft file hosting and synchronization service.
Interesting observation, but should not be the cause of my issue, as the shared folders in question worked properly as folders in File Explorer until suddenly three days ago changed into URL links without any contribution from my side or the owner.
When trying to view them online, I received a flashing immage overlaying the "this folder is empty" image saying "User migrated. User migrated. Go to my OneDrive." This only lasted for a short time and I can now find the files again in my Files." The owner of the folder then unshared and reshared one of the folders but without success. Bothe the reshared folder and the others stayed as URL links.
But we observed tht the structure of the sharing links created by the owner changed from https://1drv.ms/f/s!... for the original sharing to https://1drv.ms/f/c/... for the new sharing of the folder. But both only create URL links in File Explorer for me unlike the observation of Kudret. Interestingly, both the old and the new link for the same OneDrive folder still work online.
To me this all indicates that the issue is caused by some sort of data centre restructuring initiated by Microsoft.
Hence it is time for Microsoft support to participate here, communicate with affected users and provide a fix ASAP.