A Microsoft file hosting and synchronization service.
My wife and I experienced this issue, and we found a resolution by creating a new folder until that folder worked. We had a folder that worked for a long time up until this month that was created on my computer being the 'IT guy'. But when it deprecated to just a link and since she did most of the file saving, I decided she should have the original folder instead of myself. First attempt came across as a link only, but on our second attempt after migrating some files to the folder we got lucky and when I accepted the sync it appeared as a special folder with a chain symbol on it and displays and operates like an actual folder on my pc too. So, whatever the glitch you may just need to recreate a new sync folder until it gets past it and then cross your fingers it comes in and stays a folder until Microsoft actually focuses on fixing the issue. It's a shame its been nearly a year and still unresolved on their end. I guess home accounts are small fish compared to their corporate accounts.