Non-negative number required error while appending tables

I am experiencing the same error when refreshing my dataset, however this dataset has been refreshing without issue for over a year. I just updated to the August PowerBI yesterday and this is the first time I've tried refreshing it with the new update (I was using the July 2018 version previously).

I'm also running into a further issue where I can't select the window after the failure and have to kill the task in task manager. It's acting as though there is an extra window open when the failure occurs while I don't have the query editor open, but I can't actually see any extra windows and tabbing between available windows does not show any options available ot me.

If I trigger the failure while query editor is open I get the large failure block that looks very similar if not the same to the one pasted in the comments above.