[imp] Installation problem

[JC]

I'm trying to install horde and imp in a new server with the following items:
Debian 2.6.26-2-amd64
horde 3.2.2
imp 4.2.4
apache 2.2.9
php 5.2.6
when I try to install for the first time to configure it by web, it show the
following:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to
complete your request.
Please contact the server administrator, [no address given] and inform them of
the time the error occurred, and anything you might have done that may have
caused the error.
More information about this error may be available in the server error log.

And the log (apache2 error.log) shows the following:
[Wed Sep 30 12:31:26 2009] [error] [client 200.60.166.10] (13)Permission denied:
exec of '/var/www/horde3/login.php' failed
[Wed Sep 30 12:31:26 2009] [error] [client 200.60.166.10] Premature end of
script headers: login.php

the permissions of login.php is the following
-r--r--r-- 1 www-data www-data  8740 ene 29  2009 login.php

I hope you can help to resolve this problem