Ok, if the error is still the same, then the problem is not that mod_rewrite is inactive like the horde devs said. Otherwise you would get an apache error (most likely 500) that the directive RewriteEngine is invalid.
Do you remenmber what you did in the horde install which might have caused the "User firstname.lastname@example.org
is not authorized for Horde." error? I guess this must be something in the horde user config or changing of a databse login or similar and not a change in a apache file.