Can it be that the experience and level up mechanic breaks once level 20 is reached?
I have a character with 8 Cleric, 2 Fighter, 10 Prestige Class 1 and 1 Prestige Class 2 and the experience counter ceases to function once I added the 20th Character level. It allows for level up all the time and does list the experience as xxxxxxxxx/0 instead of the usual notation.
That is beside the point that there would be rules for epic progression:
http://paizo.com/pathfinderRPG/prd/gamemastering.html