I received an official response from Apple Developer Program Support just a few minutes ago...
TL;DR: Bump build #, rebuild, upload, and try submitting app for review again.
I ran into this issue as well. I built the app using Xcode 9.2 (9C40b), which is a full release and not beta. I attempted to submit the app for review and external testing multiple times at various points throughout the day, no luck, all failed. I was able to submit for external testing yesterday, but today things stopped working.
After submitting a bug report to Apple about this, I received a response with the suggestion to bump the build number, rebuild the app, and upload it again to iTunes Connect/TestFlight. After doing so, I was successful in submitting the app to our external testers group.