Trying the "guix pull" a short while (and one commit to Guix) later, the error is gone. Which is a bit strange: whatever the cause of the error was, it's hard to believe that adding the package "python-sqlalchemy-stubs" (which is what commit f8a4724c101880892640dcc2fe3438dc2a26b624 did) could fix it.
Konrad.
