(define (dummy-session-handler)
 (lambda (c)
  (match-let (((success details session) (find-session)))
   (if success
     (send-status 'ok (->string `(,success ,details ,session)))
     (begin (write details) (newline)
     (match details
      ('invalid-session (send-status 'not-found "invalid session"))
      ('invalid-secret (send-status 'not-found "invalid secret"))
      ('session-not-found (send-status 'not-found "session not found"))
      ((quote no-cookie) (send-status 'not-found "cookie"))
      (else (send-status '500 (conc "Failure during session handling: " details)))))))
  ))

Try these:

(let ((x ''foo)) (printf "Matching ~S\n" x)
   (match x ('foo (printf "Found foo\n"))
            (''foo (printf "Found 'foo\n"))))

(let ((x 'foo)) (printf "Matching ~S\n" x)
   (match x ('foo (printf "Found foo\n"))
            (''foo (printf "Found 'foo\n"))))

...the difference purely being in the definition of x