Preview.Error : The type of current preview value is too complex to display

Hi! 

I just encountered the same issue, with the same systemuser table. 

What worked in my case, was to add a "Removed other columns" step manually. I was interested in just two columns so I knew what I needed. Preview worked fine after adding this step. 

Also, in my case data load worked even if preview threw an error. So I was able to see the columns and to see what columns I need and what can be removed.

Best regards