Fatal error with horde3+imp4 on squeeze

Hi,

I upgraded Debian from lenny to squeeze, everything works fine except imp4,
every time I try to access imp I get:

A fatal error has occurred

imp is not activated

[line 803 of /usr/share/horde3/lib/Horde/Registry.php]

I have purged and reinstalled horde3 and imp4, with old and new
configuration files, and still the same error. I have tried to downgrade to
php 5.2 as well.

I'm using apache2-mpm-worker and php5-cgi. Anyone else is having this
problem?

Regards.
-- 
Carlos.