System Error when sorting pivot table rows by other column not used in the table

So I have been having issues with generic "System Error" that just keep appearing sometimes in my Looker Studio reports. The problem was, the report worked fine for me, but sharing it with others on the team wouldn't work. They would just see "System Error". 

I finally discovered the cause (at least in my case). In the problematic pivot tables, I was sorting the rows by another variable that was not used anywhere in the table. This breaks the pivot table for other viewers. When I changed the sorting back to being sorted by the row variable, it works again. 

Also, this is only a problem for pivot tables, not regular tables.

Posting this for posterity and everyone struggling with the same errors.