Their seems to be an easy fix - just change your password.
A bug in the latest update for Diablo III appears to be prevent a certain number of players from gaining XP.
Numerous users on the official Diablo forum who are playing a digital download version of the game have posted to say they are experiencing the problem, but at least there seems to be an easy fix.
According to users the fix simply requires you to change your password and then log in again.
Have you experienced the XP bug? Did changing your password solve the problem? Let us know in the comments below.