A Microsoft file hosting and synchronization service.
A couple of comments to add on this:
1 - despite Microsoft's claims that they don't support when one user has been migrated and the other user hasn't, both my iOS and Android devices continue to work with links to shared folders without any problems whatsoever. So they can support it - I'm starting to wonder if this is deliberate in order to force people to use internet rather than local URLs.
2 - I suspect Microsoft's plan for the 'solution' for this problem is simply to wait until the second user has also migrated, at which point everything will start to work again. Given that they've supposedly been doing this migration for over a year now (which in itself beggars belief), it's probably not a good idea to hold your breath while waiting for this to happen...
Lots of words come to mind - lunacy, idiocy, insanity, incompetence, etc.
^^ totally agree