Law of unintended consequences.
Looks like you're losing the referrer somewhere along the line, although I've yet to be able to replicate...
Could you let me know:
1) What server you're running
2) Whether you're accessing it over TLS or not
3) Confirm that it is infact the 2fa plugin or just the referrer problem by disabling the 2fa plugin temporarily on the broken version of Known, logging out and logging back in again.