The easy fix:
1. Go to the finder and hit Command + F to start a new search.
2. Choose that the name should begin with: SC Info
3. Select that it should look for folders
4. Make sure that it looks for System Files, by hitting the little PLUS sign and adding that search parameter.
When you find the SC Info folder in your Users folder under Shared (it's invisble, hence the search thing above), trash the folder, restart iTunes and you can now watch movies in all their rented glory.
Thanks to this blog entry; How to solve iTunes error 5103
