Usually that happens if the API keys were configured incorrectly (for example: mixing the live mode and test mode API keys). When you go to live mode did you check to see if everything is set for Live mode correctly? Verify the following:
1) Make sure the subscription was created for live mode. If you create test mode subscription, those can only be used for test mode. For live mode, you need to create subscription plans in live mode.
2) The plugin settings is set to live mode.
3) The Live mode API keys are set in the settings.