The following simulation illustrates a Hosted Payment Form Load error during AR invoice payment where a Braintree Connection is primary and a PayPal Connection is the fallback. 

Select an invoice to pay where the Customer Reference is ERR02.  

Click Charge with New Credit Card. Now, when the payment screen loads you will experience a simulated timeout of the primary connection.

Once the simulated timeout completes the fallback payment screen is displayed.  Process the payment.

Once the payment is complete you can see it in the fallback account, not the primary.