This issue is caused by any plugin loaded before Shopp that invokes the session_start() function.
This is because Shopp uses it’s own session handlers to manage the shopping cart session and those handlers must be set before session_start() is called. If the handlers are not set before starting the session, the cart will not function.
Try either disabling all plugins to ensure this is indeed the case, or try disabling plugins one at a time until the issue stops occurring. This will allow you to pick out which one is, for sure, causing the issue.
A good note is to also remember that certain more “complex” themes, that also function like plugins may have behaviors that make the same types of calls. Reverting the theme to a default, unmodified, twentyten/twentyeleven theme may be necessary if the behavior continues.