I don't have btrfs anywhere (only ext4).  During bisecting experiments I
used the attached system config (but the config shouldn't matter I think
as I tried various variants, and all gave me the same result).

Any idea how to dig further?  Perhaps there is something I can do in the
Guile prompt.

¹ http://git.savannah.gnu.org/cgit/guix.git/commit/?id=b1a505baf61cc771197eb44af9173f31d2bace46