Instead, I used udisksctl, a command-line interface that interacts with the udisksd service.
Here's what worked (/dev/sdb5 is the partition on my hard disk marked as crypt-luks):
udisksctl unlock -b /dev/sdb5
udisksctl mount -b /dev/mapper/ubuntu--vg-root
After typing the first command, you'll be prompted for your encryption passphrase. Once the encrypted partition is unlocked, the second command will mount it. If that's successful, you'll end up with a message similar to this:
Mounted /dev/dm-1 at /media/dpm/e8cf82c0-f0a3-41b3-ab28-1f9d23fcfa72
From there I could access the data :)
Unmount the device:
udisksctl unmount -b /dev/mapper/ubuntu--vg-root
You'll need to deactivate all logical volumes in the ubuntu-vg volume group first. Otherwise you'll get an error along the lines of 'Device busy' if you try to lock it (more info):
sudo lvchange -an ubuntu-vg
Then you'll be able to lock back the encrypted partition
udisksctl lock -b /dev/sdb5
udisksctl commands are executed without sudo.Device mapper names: the ubuntu--vg-root naming might change across Ubuntu releases (e.g. I've seen it called system-root and ubuntu-root too). An easy way to find out the name is to run the following command after unlocking the LUKS partition:
ls -la /dev/mapper
Then looking at the output of the ls command, the name you'll need will be generally the one symlinked to /dev/dm-1
Device mapper names, alternative: an alternative to the previous command is to run:
lsblk -e7
There you'll be able to see the device name mapping as a tree view. The -e 7 option is used to exclude the loop devices (ID 7) created by installed snaps from the output. Simply to have less clutter.
sudo lvs command to find out the names of volume groups and logical volumessudo lvchange -an ubuntu-vg command before you can lock it from the GUI.