TestFlight App - Error Uploading Build

I'm a very recent iOS / Objective-C developer. As such, very frequently I find myself banging my head against the wall - either due to the certificate/profile nightmare or to something as such...
Today I spent 6 hours (!) trying to figure out this error:

"Invalid IPA: The keychain-access-group in the embedded.mobileprovision and your binary don't match."

This occurred when trying to upload an AdHoc release to the TestFlight application. I'm new to this, but I had been able to do this successfully before.

I finally discovered the problem after following the steps described on this document on "How to create an IPA". For some reason, my "Archive" scheme settings were changed and had the Build Configuration set to "Release" instead of "Ad Hoc", as it should...

Thank you, TestFlightApp support.