There has been some noise on #lisp the past days about current usocket using an internal SBCL feature that's breaking with current SBCL. Try downgrading your SBCL to see if that solves the problem, and if it does, wait for an usocket update.
that's not it and it only happens with a few days old, freshly compiled sbcl.
on the other hand sbcl's network stuff is the last source of major stability issues we have with our codebase, that's why i have it on my TODO to add an option to use iolib for the network communication in cl-postgres. i've seen sub-serve-event too many times in weird backtraces...
a few days ago i had a 30 min timeframe to move cl-postgres to iolib/babel, but gave up on something. iirc, the read-a-string-until-a-zero-byte-comes was unnecessarily hard to implement with the current iolib streams (which are being worked on currently by Stelian).