I had the same issue. For some reason, the generic Mailgun set up worked for a while and then stopped. Editing the config.production.json file fixed it.
Details: Signup/login fails in Dawn theme with Portal (Beta)
I had the same issue. For some reason, the generic Mailgun set up worked for a while and then stopped. Editing the config.production.json file fixed it.
Details: Signup/login fails in Dawn theme with Portal (Beta)