I can't believe it happened but I did it...
Let's assume following scenario - we have Production org with email@example.com (that's the only user I have access to on prod).
Then we have 2 developer sandboxes - S1 with firstname.lastname@example.org and S2 with email@example.com (note that these are the same users on both sandboxes). Now I'm going to refresh S2 directly from S1 by clicking "Refresh" on prod with context of firstname.lastname@example.org.
I tried to login to S2 using original username from before refresh - doesn't work.
I tried using production username with suffix of my refreshed sandbox - doesn't work.
After the refresh is done, it seems that S2 doesn't have ANY active users... All of them are having '.invalid' suffix on their email addresses.
Anybody have any idea how to handle this?