I am not sure when and how it happened, but the loop module does not seem to be
working correctly anymore.

It worked until very recently, like 2 days ago. I cannot remember if it broke
after an update.

$ tumbleweed status
latest   : 20200923
target   : 20200128
installed: 20200923

For example:

$ sudo mount -o loop -t ntfs /mnt/dfir sda.img
mount: sda.img: failed to setup loop device for /mnt/disk.

Losetup return nothing:

$ sudo lsmod | grep loop
loop                   40960  0
$ sudo losetup
$ echo $?
0
$ ls /dev/loop*
/dev/loop0  /dev/loop1  /dev/loop2  /dev/loop3  /dev/loop4  /dev/loop5 
/dev/loop6  /dev/loop7  /dev/loop-control

Nothing wrong in dmesg:

$ dmesg | grep loop
[    0.086802] Calibrating delay loop (skipped), value calculated using timer
frequency.. 4599.93 BogoMIPS (lpj=9199872)
[  282.028818] loop: module loaded