X cannot be used is caused by some weird UI lag between you and the server or something. It most often shows up when you hit a lag spike or leave your game client open for a long time. You can tell it's in effect when Multishot's recast shows 0:12 or something after using it instead of 0:15. The icon will light up as normal but you will have to mentally autocorrect for the display discrepancy. It's annoying, but closing the game client (not just logging out, Quit at main menu too) can help reset the delay.

It's been an existing problem for quite some time now, not sure if the fault lies with the game or one's internet connection/PC specs.