[Zope] Problem with dtml-if sequence-start

[Borja Prieto]

I have a problem with the following code, wich is a DTML methos that
processes a form:

<dtml-with REQUEST>
(First, I check if the user has completed the txtCertificado field)
   <dtml-unless txtCertificado>
       <dtml-return "RESPONSE.redirect(URL1+'/InicioIC'+'?msg_error=Error:
debe introducir un número de certificado')">
   </dtml-unless>

(Then, I lookup in a MySQL table with the method BuscaCertificado if a
record with the value txtCertificado does already exists)
   <dtml-in BuscaCertificado>
(if it is found, return with an error message)
        <dtml-if sequence-start>
            <dtml-return
"RESPONSE.redirect(URL1+'/InicioIC'+'?msg_error=Error: el certificado
'+txtCertificado+' ya existe')">
        </dtml-if sequence-start>
(and if not, go to IC page. This is the part that does not work)
        <dtml-return
"RESPONSE.redirect(URL1+'/IC'+'?msg_error='+txtCertificado)">
   </dtml-in>
</dtml-with>

The problem is that the program, though evaluates right the cases for
nonexistent parameter or parameter already in the DB, it does not even reach
the line:
<dtml-return "RESPONSE.redirect(URL1+'/IC'+'?msg_error='+txtCertificado)">
Instead, it shows a blank page, with no code. There is no problem with this
code, because I have tried changing it to show a simple message, and it
doesn't work, too.
Please, ¿could anyone tell me what I am doing wrong?
Thanks in advance


Borja Prieto Ursúa
Alanta