Two things going on here: for the water to back up, there must be a clog in the line below the toilet - that needs to be addressed as they only get worse with time; second, even if there were a clog, unless the water is rising in the toilet and overflowing the bowl, it is probably the seal between the unifit adapter and the flange. Yes, you set the unifit adapter with a standard wax or waxless ring. The rubber gasket that seals the toilet to the unifit adapter probably is replaceable, but shouldn't be necessary unless it was damaged. You'd not normally notice anything if there wasn't a clog somewhere. My unprofessional opinion.