[horde] A second login for IMP ?

[Tô]

Hi all,

I have two Horde's installs working over a Kolab server :
1. Horde 3.2-CVS with Imp H3 3.2-CVS
2. Horde 3.1.3 with Imp H3 4.1.3

Both are configured to use the same LDAP server as authentication 
backend, with exactly the same parameters.
In the first install, I log in once and access directly to Imp. In the 
second, I have to log in a second time to access to Imp.
First question : is it due to the version of the modules ?

When I setup the second install to use "Let a horde application handle 
authentication" (and choose Imp), I no more have any users to set 
permissions to...
Second question : is there's a way to deal with this ?

Thanks in advance for all suggestions,

Christophe