New install - ERROR 500 when adding subcategory or product

[mosleyh]

I have just made a fresh install of 1.2.4. I set all permissions as instructed in the wiki, and added a .htaccess file to turn php5 on for 1&1;web hosting. Having had trouble with importing categories and not finding any thorough documentation of the process, I decided to add them manually for now. However, when I click the "Add a subcategory" link, I get an Error 500 - Internal server error. What might I be missing? (EDIT) I get the same result when selecting "Add a product"