Imagecache help needed please

Hi all,
I have been looking through the forums for ages on this one - tried lots of things but nothing had cured the problem so just thought id ask to see if anyone can see something i've missed or shed some light on this frustrating problem...thanks!

Running Drupal 6.13 - Im using imagecache with GD to process images. Galleria is used to push the images there. If I turn the imagecache functionality off in Galleria, the Gallery works ok, but when turned on and presets assigned the images cant be found.

I can make presets in Imagecache, but the image cache tester image does not display. When clicked it returns a '500' internal server error. Imagecache makes its folder, but does not make folders corresponding to the presets.

The permissions on the files dir and the imagecache + tmp is 777

The strange thing is i am running an almost identical drupal install for a different client at a different www, but with the same web host and service plan...It is on Drupal 6.12 and has slightly different module setup, but the gallery/imagecache works perfectly...

The htaccess files are the same for both sites, so i am guessing it cannot be a problem with the host/rewrites etc??

The only thing i did differently was i put the imagecache folder in the wrong folder (default/modules instead of all/modules) at the beginning - i wasnt thinking! but I uninstalled the imagecache module and removed all traces from the DB before re installing and finding the same problem...

Could it be a module conflict? I have the following other modules running: frontpage/cck/simplenews/wysiwygAPI with tiny mce/ IMCE/IMCE Bridge/Statcounter/Transliteration/Simple paypal framework/recent comments/donation + donation thermometer/countdown timer...

Many thanks for your time,

have a good day, joe