DS: The return URL could not be verified for Service Provider

[jehan procaccia]

hello,

I upgraded my SP to shibboleth-2.4.3-3.1.i686 on a  centos6 plateform .
I configured a SessionInitiator DS for a specific application this way:
<SessionInitiator type="Chaining" Location="/DS" id="DS" 
isDefault="true" relayState="cookie">
<SessionInitiator type="SAML2" acsIndex="1" 
template="bindingTemplate.html"/>
<SessionInitiator type="Shib1" acsIndex="5"/>
<SessionInitiator type="SAMLDS" 
URL="https://shibds.it-sudparis.eu/wayfdsit/WAYF.php"/>
</SessionInitiator>

when I try to login I immediatly end up on the DS responding on the browser:

Error: Invalid Query
The return URL 'https://www-public.it-sudparis.eu/Shibboleth.sso/DS' 
could not be verified
for Service Provider 'https://www-public.it-sudparis.eu/shibboleth'.

I cannot figure out what is wrong, is it the SP session initiator ?, the 
metadata ?,  switch DS implementation ?
or the way the application (dokuwiki) initiates the session:
<a 
href='https://www-public.it-sudparis.eu/Shibboleth.sso/DS?target=http://www-public.it-sudparis.eu/~procacci/dok/'>Login</a>

on the same sever, an another dokuwiki instance calling an older WAYF, 
works file
<!-- manually defined additional DS initiators - without isDefault and 
with custom IDs and Locations -->
<SessionInitiator type="Chaining" Location="/WAYFMT" id="WAYFMT" 
relayState="cookie">
<SessionInitiator type="SAML2" acsIndex="1" 
template="bindingTemplate.html"/>
<SessionInitiator type="Shib1" acsIndex="5"/>
<SessionInitiator type="WAYF" acsIndex="5" 
URL="https://shibidp1.it-sudparis.eu/WAYFMT/WAYF.php"/>
</SessionInitiator>

thanks for your help .