Another example of error :
Error unlocking /dev/sdb2: Error spawning command-line `cryptsetup luksOpen "/dev/sdb2" "luks-ac0dc77b-68a4-...." ': Failed to execute child process "cryptsetup" (No such file or directory) (g-exec-error-quark, 8)
Solution :
Install cryptsetup
sudo apt-get install cryptsetup