Had a similar error trying to use Fabric’s Twitter Kit
Error:Execution failed for task ':app:fabricCleanupResourcesDevDebug'.
> Crashlytics Developer Tools error.
Detailed error
ERROR – Crashlytics Developer Tools error.
java.lang.IllegalArgumentException: Crashlytics found an invalid API key: XXXXXXXXX.
Check the Crashlytics plugin to make sure that the application has been added successfully!
Contact support@fabric.io for assistance.
After login in Fabric, download the AndroidStudio plugin and let it configure everything all worked fine.
(Btw, I really don’t like this setup flow)
EDIT: It also can be done without install the AndroidStudio plugin. Follow these instructions from the Fabric site https://fabric.io/downloads/gradle