Hi Chad Thank you for your detailed report.
Before I dig more into it, can you double check there is no other instance of rrdcached running, and using the same socket location? I think rrdcached was disabled by default on jessie, and was enabled by default on stretch. And since you are using the default rrdcached socket location /var/run/rrdcached.sock there might be a conflict there.
Description: This is a digitally signed message part.