Help! 500 Error when trying to create Adoptables, Items, Shops, Links?

[Storyteller]

Hey there, me again ^^;;;

I just did a fresh install of the Mysidia script (using this tutorial, except, with my own host), because I absolutely couldn't figure out why it wasn't letting me add links and wasn't showing the security question at sign up.

While that solved the security question problem, I now can't add /anything/ to my site. I can upload images and adjust settings like breeding and trading just fine, but if I click "Add Adoptable/Shop/Link/Item" I get this:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, cgiadmin@yourhostingaccount.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

Since this is a fresh install, with only edits that I had going in my last install of Mysidia (when at least shops and Adoptables were working fine), does anyone have any ideas where I went wrong? From the symptoms, does it sound like I skipped a step somewhere?