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

Use the potions again, but let them expire on their own.

I had this happen to me a few months ago, so I just used the potions, set up shop and went to sleep. The message went away after that.

A quicker solution is just to restart your whole computer. Logging in and out doesn't work. Probably closing the client completely and rebooting it will have the same effect as restarting computer, but don't remember.