Thanks, utu, I haven't done this yet, but I'm considering it.

Meanwhile I'm going through the mailing list and found something:

https://lists.debian.org/debian-knop.../msg00000.html - title: "losetup from 7.6.1 can't detach a cloop device" - sounds familiar, isn't it? Seems like this is caused by a change in losetup. Klaus Knopper posted an answer in which he mentioned that he filed a bug report and using the losetup of busybox should work. I can't test that right now, just leaving the info here so it will not get forgotten.