[plt-scheme] send/suspend + sandbox + exceptions = strange thing

A week ago I asked about how to integrate send/suspend with sandbox.
"send/suspend" is for web, it use continuations
Sandboxes broke continuations, but I need send/suspend which can work with multiple sandboxes.

I wrote my own "send/suspend!", using exceptions: in sandbox it raise exception with argument for send/suspend, and running eval with exception handler allow to call usual send/suspend otside of sandbox

(define send/suspend! (lambda (f)
  ;;prepare f for out-of-sandbox                      
  (raise (list 'send/suspend! f))))


;;predicate for send/suspend! exception
(define exn:Socium-eval:send/suspend!? (lambda (exn)
  (and (list? exn) 
       (not (empty? exn)) 
       (equal? (car exn) 'send/suspend!))))
  
;; "send/suspend!" exception handler
(define exn-handler:send/suspend! (lambda (exn)
  ;;(lambda (k-url) ...) for "send/suspend"
  (define f (cadr exn))
  ;;make usual "send/suspend"
  ;;return to continuation with send/suspend value
  ;;(jump in sandbox again)
  ;;this value will be on the place of exception
  (send/suspend f)
  ))
  

(define sandbox-eval ( ............
      ............
      ............
  (define e (personal-evaluator e-mail))                    
  ;;eval expr 
  ;;(with exceptions handling)
  (with-handlers ([exn:Socium-eval:send/suspend!? exn-handler:send/suspend!])
    (e expr))
      ............
      ............
))


and got some strange trouble: when I run my web application, with just one call of "send/suspend!", I see some web page what I need, 
but when I press "submit" there, I got error:


broke the contract 
  (->*
   ((-> request? response?))
   ... ... ... ... ...
   )
 on serve/servlet; expected <response?>, given: #<request>


I started to discover, where program breaks ...
it happens later than exn-handler:send/suspend! , but before returning to send/suspend! from exception handler

if I put message "hello world" instead of send/suspend returned #<request> in exn-handler:send/suspend!, I really can see it in browser instead of page

how can it be, what is it?