FYI For anyone who gets the above failure (Test Failed. Exception of type 'SolarWinds.ApiProxyFactory.TransientApiProxyException' was thrown.) on an SNMP test this seems to be (in a couple cases I've had in different organizations, installations) an issue with NIC priority in systems with multiple NICs. Changing priority doesn't always fix it but disabling the interface that doesn't have your poller's primary IP does.
You may also see that your poller is not syncing with the database or that it is not 'up'.
This has only happened to me with VMWare boxes but, FWIW, I've only installed SolarWinds on VMWare VMs.
I've had this happen without doing anything explicit with the box, sometimes it seems like performing windows updates can do this. I've never had sole administrative access to the systems so I can't verify that another administrator didn't come in and change something or enable a NIC that was disabled but it seems like this has also happened.