No, this error is not reproduced in SBCL. The file is compiled by the correct version of SBCL, which is why I thought that SBCL version could be hardcoded somewhere in Stumpwm. It doesn't seem to be the case, of course. Apparently, I'm struggling with my own absentmindedness and the problem is mine. Sorry.
When I enable Swank in config, I get a strange message, saying that
file /usr/lib/sbcl/contrib/sb-cltl2.fasl "is a fasl file compiled with
SBCL 2.4.7.debian, and can't be loaded into SBCL 2.3.7.debian".
If you run SBCL in the terminal and try to quickload SWANK or SLYNK (or
just require sb-cltl2) do you get a similar error?
If you do get the same error could you try removing SBCL from your
system (and ensuring the files are gone) and then reinstalling it? On my
system (void linux) the file `/usr/lib/sbcl/contrib/sb-cltl2.fasl` is
provided by the package `sbcl`. I'm unfamiliar with debian, but can you
use the package manager to find out what package provides that file and
see if it its version matches up with everything else?
--szos