Cannot mount dislocker file via loopback mount

Question

I am trying to un-encrypt Windows 10 Bitlocker encrypted drives using dislocker. I downgraded my desktop to Windows 7 and only then found that Win7 cannot unlock the newer version of Bitlocker that Win10 uses. So, I found out about dislocker and installed it from the AUR on my Arch Linux laptop. I have the password and the recovery keys, and dislocker (FUSE flavor; the latest version: v0.7.1) appears to unlock the drives:

$ dislocker -v -u -V /dev/sdb1 /aur/disk1/
Enter the user password: ****
$ mount | grep dislock
dislocker on /aur/disk1 type fuse.dislocker(rw,nosuid,nodev,relatime,user_id=1000,group_id=100)

When trying to mount the dislocker-file, I get this error:

$ sudo mount -v -o loop /aur/disk1/dislocker-file /mnt/iso/
mount: /mnt/iso/: failed to setup loop device for /aur/disk1/dislocker-file.

That didn't seem dislocker specific so I diagnosed the loop option. The loop kernel module is loaded. Creating and mounting an iso via mkisofs works fine. I see no other output from kernel or systemd (dmesg or journalctl).