Generally, what you should grind always depends on the time you have. If you have a lot of time, grind snakeskins on epic, because you get the mats and decent EXP.
If you only have a little bit of time, Haunted Citadel sounds good. I tend to run the highest dungeon I possibly can before I leave the game for a while, as long as I can finish it in at least two rounds. Knights get empty, load up while I'm away, can finish the dungeon - exp in little time.
@nomad
Talk to support. There's no other way.