First, I would hope you have already checked but MITES. I had a BRB many years (15) ago that came in with mites. There was a mite bite either on or at the edge of the eye which became infected. Similar symptoms to what you describe. Was treated by the vet and was fine.
Things to look for:
dead mites in the water - looks like black pepper.
Since your snake is soaking, the mites will migrate to the part that is out of water, the head. Gently spread the skin especially on the lower jaw (more pliable) looking in the crevices between the scales and the skin. Again looking for the "black pepper flakes" Don't need to find more than one, if there's one, that's too many.
Second, VETERINARIAN, I can't stress enough how little we know and can help without seeing the animal. Obviously not feasable to bring to us but a good reptile vet in your area would know MORE, and have access to whatever meds needed. I see many posts on this and the regular boa forum asking about a apparently ill snake. If you think it's ill, take it to the doctor.
Please don't take any offense to my ranting, it's just I see this way to often and seems there are folks out there who think buying the snake should be the only significant outlay of cash next to the habitat. The vet isn't cheap but is a necessary occasional expense.
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Thanks,
Dave "Rainbows-R-Us"
0.1 Wife
0.2 kids
4.12 Brazilian Rainbow Boa
1.1 Ball python
0.1 BCI "Elvira" normal from 1989
1.0 BCI albino / het-anery
0.1 BCI Hypo / het-albino
0.1 BCI Anery / het-albino
0.1 BCI Hypo (possible super)
lots.lots.lots feeder mice and rats 