How do I fix this shopping cart error?

Just upgraded to v1.3.9f (this is the newest my hosting site offers), but this problem was evident in the old version, as well.

When a customer places an item in the shopping cart, they get the error:

Server error
The website encountered an error while retrieving It may be down for maintenance or configured incorrectly.
Here are some suggestions:

Reload this webpage later.

HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.

This is using Chrome. On Firefox, customers get a blank page.

I've done a search and found a few things that could answer my question but not how to fix it. Help!