‘Couldn’t fold the expression to the data source’

Hi all, 

I've built a report based on a SQL db, published to BI service via an on-prem gateway. 

When I view it online the top two visuals on my report page are fine and the bottom two have 'error cannot be loaded' - same data source but the bottom has a report filter. If we switch to Import mode it displays correctly, switching back to DirectQuery it once again errors, help! 

The subject line of this post is the error we've managed to get from a trace we applied, which Microsoft analysed for us but it's gibberish to me as I don't understand how I need to apply any fix when two of my charts are fine and two other aren't, and it's the same data source going through the same gateway?!