A Microsoft file hosting and synchronization service.
I'm IT consultant, and I know this started happening since last year, because my customer asked me to resolve this issue, but at that time, I just told them this was Microsoft bug and all we could do was just waiting for patch release.
Finally, this issue happened on my computer today for my family's shared folder, and found Microsoft has not solved this yet.
I have to recommend to customers to switch to Google Drive if they want to keep shared drive which can be opened in explorer.
I started to click 'I have the same question' link in every single similar question in Microsoft community, both in English and Japanese, to raise up this issue to Microsoft.
I hope more and more people join.
FUZAKENNA MS!!!! (<- Very angry word in Japanese)