Error Uploading App to Marketplace

I'm trying to upload an app to the Zendesk Marketplace. I've fixed all other errors (image size, etc, manifest file).  When I try to upload, I get the following error: 

1 error must be addressed before proceeding
Error: Something went wrong when trying to validate your app. Please try again or contact Zendesk Support:

Also, there's a toast message in the upper-right corner that says "Unexpected Token T. resource... not a valid JSON".

Could you help me figure out what is going on? Thanks!