A Microsoft file hosting and synchronization service.
I was able to fix this just by renaming the ROOT folder in the library (ie. append a "1") then undo the change you made. I was able to successfully open all files without issues after. The only downside I see to this might be that the metadata on the root folder will change. Subfolders should not be impacted.
Obviously depending on your business this will have to be done on off-peak hours, or scheduled outage.