Buzzing acoustic string

My first and only acoustic guitar is an upgraded inexpensive guitar. I
put in a new nut, new tuners, a new saddle and new strings. No big
deal, but it sounds a lot better, except for one thing! The 5th string
buzzes when I use a capo, and the one piece I want to learn on this
one guitar requires a capo on the second fret. I have checked the nut,
and for the 5th string, it is as perfect as I could get it, and far
better than any other guitar I have. I have worked on the saddle, and
it is again as good as I can get it, a nice bone saddle, FWIW. I
raised the action a bit, and it still buzzes. Tried a different capo,
buzz city. Only the 5th string does this, everything else is OK, not
great mind you, but it works.

So I figure maybe it's the string itself...but...could it be something