I had the same issue and followed the same steps above before calling support to get the issue fixed.

Eventually i was asked to to access https://API_URL/MetadataTransformService/hc  from the console server. 

What happened for me was the certificate the DS server provided to the console server was being tagged as invalid and blocking access.  It was providing an internal cert vs. external cert

Fortunately after discovery of this we had to update the main UEM cert due to expiry, and took the time to remove the internal cert and standardize on the external cert with modified host files on all services (DS, console and UAGs)