A Microsoft file hosting and synchronization service.
This problem now occurs on my Windows 10 machine too. Effectively no 2FA, unless Microsoft believe 2 x 1FA is 2FA.
It's very frustrating to have a working product undone in this manner, and I feel I now have to look at alternate methods to achieve the purpose of the Vault.
To be clear, in my case proper 2FA in not available as an alternate method, it's PIN or fingerprint.