Bent String at Nut
A rare cause, but it can happen
If you put your capo on too close to the nut, you can bend one or more strings downward enough to cause a first fret buzz:
See that dip the string takes as it exits the nut? This low E string is tuned to pitch, and the deflection is just enough to make the string rattle against the first fret.
It's not a common problem, but it can happen. A new string will clear up the buzz right away, of course.