Internal Server Error when trying to create classifieds listing

Well the title of this post is pretty self explanatory.

When trying to create a classifieds listing i get a Internal Server Error when i press submit. I assume this has to do with the featured image part and assume its because of a server setting. However im new to server admin so need a little guidance.

I am using a dedicated root server so have full access to everything as its self managed with plesk 10 (i know that is a swear word here lol).

Any ideas???

If it helps i also received an error when i tried uploading a plugin the other day that was 800kb however any smaller is fine. Dont know if that helps.