Thanks for being such a long-time user of ODK and welcome to the forum @sokwetz! Also thanks for adding your profile picture as it helps build community. We're glad you're here. When you get a chance, please introduce yourself on this forum thread.
I don't have an answer to the exact error message you're seeing. I know there are still some users of Aggregate so maybe someone on the forum will have advice. Unfortunately, Google App Engine support is now legacy and only available in Aggregate v1.x as noted in the ODK documentation and further explained in this post on Upcoming changes to Aggregate from January 2019. For new projects, we recommend checking out ODK Central.