@mowoolli I had this error today using Postman but it's a simple thing to solve. Just add the header Content-Type:application/json to your request. That's all :).
I'm giving a try to the free demo API but all forecasts come in a 3 hour ranges. This happens using the gfs and iconEu models.
The proffessional APi does have hourly forecasts? Or is it all the same, having just 3 hour ranges?