Hi,
In the repository version of CLISP, there's been some changes to the condition class hierarchy. usocket/backend/clisp.lisp line 685 reads:
(system::simple-os-error
Changing that to:
(ext:os-error
Should work for the current version (2.49 - simple-os-error inherited from os-error and the usocket condition argument accessors are otherwise generic) and the upcoming CLISP version.
Vladimir