Why do I see a "URL No Longer Exists" message when opening a Soapbox Mailer campaign record?

When opening a Soapbox Mailer Campaign record page, or after saving a new Soapbox Mailer Campaign record, you might receive an error page that has the following error message on it:  "URL No Longer Exists".  Here are the reasons why you might see this page.

1) In Salesforce, you might have clickjack protection enabled for non-setup custom Visualforce pages. You can turn this off by going to Setup > Administration Setup > Security Controls > Session Settings > Clickjack Protection:  ensure that the "Enable clickjack protection for non-setup customer Visualforce pages" checkbox is unchecked, and click the Save button.