Darn small pics.. lol Looks like the wall has grown down from there with this indent/crack for some time. It can take up to a year to fully grow out a hoofwall..
I have to assume that this abscess may have a foreign body in it that is aggravating the situation. Perhaps even a bonechip.. You may need to get some kind of xray to find that out.
As bfhooves mentioned abscesses at the c-band are often harder to "cure" as the pus has to travel up to get out. I would try a good drawing poltice on that site for a while. See if that helps.. XXX
And I agree about the rings, something is going on with this horses metabolism.. Light cases of laminitis(which can certainly cause abscesses) or abrupt changes in rich feed. I know, been there, done that.. I would remove all sugars possible from this horses feed program. NO sweet feed etc..
If you can get it where you are I'd recommend Kool & Kalm. Check in at http://aussielogic.com
If that doesn't work just google it.. I am feeding this to my old appy as he is pretty much blind and has trouble finding the hay and whatever grass is still around. It is a stand along feed, just add hay etc. Never add Grain to this feed. It also can be fed free choice, btw..