Dataflows - gateway not working

Hi all,

I'm querying data from an on-premise sharepoint site using a gateway for schedule refresh. This has worked really well for a few months with the dataset.

Now, I'm moving the queries to Dataflows so other people can use the cleaned data in their reports. The problem is that the gateway  throws the following error: 

I've always had problems to use gateways in Dataflows. In fact, this morning is the first time it works but after I made further changes to the queries it stopped working and can't managed to make it run again so I decided I would ask around here...

Any suggestions?