root@ubuntu-mate:~# mount -l | grep sda
root@ubuntu-mate:~# sudo cryptsetup luksOpen /dev/sda2 mnt 
Enter passphrase for /dev/sda2: 
Cannot use device /dev/sda2 which is in use (already mapped or mounted).
root@ubuntu-mate:~# sudo mount /dev/mapper/
control                                    luks-073f4f18-c0b1-41ae-8817-25c0f36bd8c3  
root@ubuntu-mate:~# sudo mount /dev/mapper/luks-073f4f18-c0b1-41ae-8817-25c0f36bd8c3 /media/test/

# Hier steht er jetzt schon seit Ewigkeiten!
# Allerdings gerade im Logfile gefunden, an diversen Stellen immer wieder:

[  840.407576] INFO: task btrfs-transacti:2454 blocked for more than 120 seconds.
[  840.407579]       Not tainted 3.19.0-15-generic #15-Ubuntu
[  840.407581] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.