Bug#853927: debian-installer: Hang in os-prober in "dmsetup create -r osprober-linux-sda1"
right after replying to the bug report I decided to re-install, just for
the fun of it. This obviously deleted the install logs, sorry.
But this time I did not have the microSD card in my internal card reader
like I usually do (/dev/mmcblk1 from my fdisk output) and the
installation finished without os-prober getting stuck. So my guess is
that the SD card may have been the culprit even though it does not
contain any OS, just a plain ext4 file system (no partition table) that
has some mp4 files on it.
If you want me to I could re-install once again, with the SD card
inserted, and check the logs. Let me know if that would be of any use.