An operation is already pending error is very generic and many things can cause this. (other answers don't relate to my problem).
I have a Luks encrypted volume that I clicked to open, a password prompt appeared, and I waited too long to enter the password. (It normally works just fine.) Now, whenever I try to open it, I just get that error instead of the password prompt.
I'm sure rebooting the computer would fix it, but I don't want to do that... I'm sure it's just some sort of mount lock that needs to be manually released. How do I release the lock manually so I can try again?
The OS is Mate on Mint 17.3.