As an update, I've reached back to the mothership, and it appears that if these conditions exist:
1) You install vRA using the reference HA (high availability) design
2) You run the vRA Configuration Wizard from the VAMI
3) In the "Agents" screen you enter your EndPoint Names, but you misspell only ONE of the TWO Proxy Agents ENDPOINT NAMES; one endpoint name is correct and one is not
4) When you check the VRA* SQL database table ado.Agents you see that the Proxy Agent in question shows the Endpoint Name as the misspelled name
5) When you attempt to add an Endpoint to vRA in the vRA console using the non-misspelled name you get "Test connection failed..."
6) When you use the misspelled Proxy Agent Endpoint Name the test connection works!
…then you have a problem that uninstalling the Proxy Agents from both IaaS Agent servers does not appear to fix.
I have not tried to edit the table on the database, as that would be a clearly unsupported by VMware.
However, there is an issue with borking the Endpoint Name for your Agents during the Configuration Wizard portion of the installation process. Specifically, when you only mess up one of the two HA proxy agents…