If you are getting an error similar to this:

"An error occurred while processing your transaction: PayPal DoDirectPayment returned error code 10002: Security header is not valid"

The issue probably has something to do with your stored paypal credentials.  This sometimes happens during an upgrade process because of how the upgrade changes the character-set of the database.

Luckily, the fix is simple.  Simply log into your paypal account, delete your api credentials, and then re-create them.  Use these new credentials with Squirrelcart and you should be good to go.