Error message from API call between Salesforce and Infusionsoft

When making a call to Infusionsoft through REST Api from Salesforce
I occasionally get the following error:

caused by: System.CalloutException: Unexpected end of file from server

You can see examples on the two images below.

Can anyone tell me how to fix that or why it’s happening?

I don’t believe any of those callouts are Infusionsoft related (I don’t recognize them anyway). That would leave SalseForce reporting a failure in making the call?

Yep, that’s definitely not an exception on our side. Maybe post it on the Salesforce Developer Forum?