"Failed to use the skill due to an unknown reason."

Whenever I've had this, unequipping every 'summon' type item (android, pets, totems, ring) and then rebooting my computer has fixed it. It has happened without auto buff gems being on my character though so my conclusion earlier on in this thread isn't correct.

I think this is in some way related to the damage glitch, not sure how though, but the two seem to happen together.. however it could just be me =/