Project Web App

I'm getting the following error message when trying to refresh a report in Excel, using an oData connection:

"We couldn't refresh the connection 'DataFeed_1_xx-sharepoint-com Timesheets'. Here's the error message we got: The underlying connection was closed: An unexpected error occurred on a send. Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host. An existing connection was forcibly closed by the remote host."

What do I need to check or fix to be able to successfully refresh a Project Web App report in Excel?