A Microsoft file hosting and synchronization service.
Thanks for that link, Mark. It supports my earlier suspicion that an ongoing account migration might be responsible for that issue.
What is sad about this is that Microsoft apparently doesn't migrate accounts in logical groups. For example, we have three accounts in our 365 Family subscription. By migrating these all at once, or at least closely together, this issue would have been minimised or even avoided for us, and for probably many other affected subscribers in similar situations.
If accounts are shared widely, it won't be possible to avoid that issue, I get that. For all closed groups of just a few family accounts, or accounts that are shared with just a few others, it would have made sense to keep these together in the migration. Too bad that no attention was given to that.