I just tested the latest version of the plugin and I was unable to reproduce any issues. This is my yaml configuration:
secrets: [ token ]
I created a service account in google here with proper access to publish images. I added the token to drone via the user interface. It is a plain text, json value (I did not base64 encode).
I ran the build and everything worked as expected:
As I am unable to reproduce, and I have many enterprise clients using this plugin in production, I do not believe there is much more I can do to assist here.
I recommend double checking your service account permissions and your drone configuration. I then recommend downloading and running the plugin locally from the command line, as described in the README, and digging into the code if you continue to experience issues.