First time duplicate folders

HiDeleted, thanks for your response. Unfortunately, the troubleshooter does not work, I can't execute it on Win10 1709 (tried on multiple machines). When you speak of "synching shared folders", are you referring to any folder in a document library with unique permissions? I checked a folder that carries the computername extension but it doesn't have unique permissions. Its parent folder however has.

I should also point out that the contents of the folders are not duplicated, only the folder itself. For example, the original folder "Meetings" now has several copies, such as "Meetings-ssdf245", "Meetings-ssdf333" and so on. The contents of the original "Meetings" folder are spread across those duplicates meaning that for example "Meetings-ssdf245" contains "File1", "Meetings-ssdf333" contains "File2" and the original "Meetings" folder contains "File3".

This is so weird and we cannot figure out what's causing it and what logic it follows. What we think to have determined though is that it is not a re-occurring event. All changes seemed to have happen yesterday within a couple of hours.

I also suspected that the site collection might have run out of storage but not so.

Any more ideas? Thanks.