Android release build error: "The value for this property cannot be changed any further."

Ran into this unhelpful error message while building a release build:

Execution failed for task ':app:processProductionReleaseMetadata'.
> The value for this property cannot be changed any further.

• And that's it, that's the whole error message. Nothing else to go on.
• Note that here "Production" is the productFlavor and "Release" is the build type. (I emptied out the configurations for both productFlavor and buildType so these blocks are empty and still getting this error)
• The dev builds are working fine
• Tried, cleaning/invalidating caches/restarting/re-installing AS/Gradle - still nothing

I recently updated this android app after one year, upgraded to Android 4.1, latest gradle, plugin versions, libraries etc. So, somewhere something is screwing up this "processMetadata" task.

Any ideas? Where could this be coming from? Manifest? Conflicting dependencies? Some incompatible configuration? Too recent version of AS/Gradle?

I'm on: Android 4.1, Gradle 6.7, Gradle plugin 4.1.1

from Android release build error: "The value for this property cannot be changed any further."