I've seen this popping back up again lately. It fixes itself if I camp and then come back in game. It seems like it only happens when I am doing quests that appear as Red.

Race? And what level is your character? The mobs are red to you?

What zone did this happen in? And what city did you use?

I think what happens is once javascript has a critical error that derails the game, xp is no longer calculated correctly. If you notice this again let me know. It may be worth watching the console for a red error message to see if you can isolate the problem.