Having a similar issue. It seems to be related to issues with the template but it is difficult to debug. Did you get anywhere with this problem?
Hi Dan, it mysteriously stopped happening for me (not related to any Python API updates or code changes on my end). I would add though that mine would return None for a particular data point/template combination, then subsequently work for the same report, so the template wasn't reliably breaking anything. Perhaps there's some bunk error handling going on on the AGOL side that returns None for a class of issues.
I also encountered a case at one point where None would be returned if your filters cause an empty result, but that's somewhat more expected.
By the way-- wouldn't suggest going through support unless you have a surefire way to reproduce it.
Thanks Brian, i think I figured out at least part of the problem is that your Enterprise needs to be externally accessible as the survey123.arcgis.com reporting service actually needs to query it.