Mine did that exact same thing. And I managed to put it back together without taking the bike apart. Thank God!!!
I squeezed in there to tighten those header nuts, too, but was unable to get to that particular header (#2 from the right).
Maybe MGvaleri did the same and this was the consequence.
Scaffolder, how did you manage to put it back together without taking the bike apart?
(Did it involve your baby?)