Paypal Payment Processor not connecting with CiviCRM
Solved:

It turns out that the CiviCRM logs contained error messages that indicated a missing database column, specifically a CiviCRM custom field. It's not clear why the column was missing, and the CiviCRM admin continued to display the field. An attempt to edit and save the custom field, in the CiviCRM admin, resulted in the same error as occured when submitting the donation form, which appeared to confirm that the custom field was the culprit. Sure enough, once this column was rebuilt, the problem was solved.