Same issue here. Every model which connects to a dataflow is failing. Based on the first comment regarding legacy dataflows... I took a simple model in which had a connection to a web based connector (sharepoint excel file) and 2 connections to dataflows (both legacy) created 2 new connections to those dataflows using the non-legacy connector, got the M code, copied it over my existing queries, republished and same error. I was assuming this was an issue with the legacy connector, but thats not the case, unless there is some cache, I'm unaware of. This is only happening in the service and not on desktop.
We updated the gateway intially thinking that was the issue and we had sucessful refreshes right after, but then they began failing again. Completley reinstalled the gateway and also no luck.