Create dataflow in power bi services errors with Unexpected Error

I have a on premise gateway that has a Web datasource connection using url abc123 that a colleague has created, which works fine. They have given me permissions to the gateway as well as the datasource connection within Power BI Services > manage gateways.

I then try to create a new Web API entity in a new dataflow in Power BI Services, I use the same URL of abc123 (but with different query parameters), the On Premise data gateway and basic authentication auto populates as it should because it sees I have access to that existing datasource connection via the gateway. I then click Next and I get the error "Unexpected Error".  

If I try these steps again but with a brand new URL (and no existing matching gateway datasource), it works and I am able to pull data into the new dataflow. What are next steps to troubleshoot this?