error 500 when using agrovoc field module

i have a drupal installation that is working perfectly. i recently installed the agrovocfield module and the agrovoc module. i followed the readme.txt instructions found in the agrovocfield folder. i used the story content type to create my agrovoc field which i called "Agrovoc Term". then i tried to use it by going to create content>story and adding a word in the "Agrovoc Term" field but i am getting the error message "An HTTP error 500 occurred.
http://localhost/agrovocfield/autocomplete/field_agrovocsubject_all" whenever i try and type in a word.

the rest of the drupal installation works fine.