I have had some improvements (I've been suffering from this on and off) after I did a chmod of the /home/httpd/html/horde folder (and subfolders)
chmod -R ugo+xr /home/httpd/html/horde
Before, only IMP was working (at all!) now, Kronolith, nag, mnemo and the horde summary are all working...
I know that some folders (like the config folders) were set to more restrictive permissions (no read or execute to 'other' and maybe even 'group') but I will live with it.
It's weird that there should be such issues, and weirder that a month or two ago, all other horde modules were working just peachily.
Odd.
G