Symptoms

'Site Collection URL not valid' error when trying to activate on a valid URL for Development instance. Production instance has no issues.
 

Diagnoses

The underlying issue was actually a '401 Unauthorized' error. During activation, Appit 1.4 authentication errors don't surface properly when the app is accessed via the App Catalog's Site Contents. The error shows correctly when clicking on the app in the inactivated site collection's Site Contents.
 

Resolution

We logged in with a Global Admin account that was previously authenticated with the Development instance and activation continued correctly.