Unable to submit Online Case Form, receive "Invalid Accounting Key 141"

Scenario:

1. Customer create basic Online Case Form. No Scripts, workflows or Website is set up.

2. Select External tab > Publishable Form URL > click link.

3. Answer fields and click Submit.

Actual Result: "Invalid Accounting Key 141"

Recommendation:

1. Navigate to Setup > Support > Support Preferences.

2. Select External tab.

3. Anonymous Customer Placeholder = select (or create) Anonymous Customer.

4. Click Save.