Hi Dean,
It seems more likely that the test has maxed out the appliance's memory. This is a bit surprising, because test connection should not typically require much memory. A few questions that may help us understand the situation:
1. How much memory is available on the appliance before running the test?
2. Are you able to provide a screenshot or a bit more detail about where the java.lang.OutOfMemoryError appears?
3. How large is the JSON API response returned for the Test Connection endpoint? This is the endpoint defined in MP Builder under Source > Test Connection Request.
Best regards,
Mark