Hey Andreas,
not really sure what you’re doing, but please ensure not to assign customer and agent role at the same time to an account. (This shouldn’t be possible, but who knows…)
Also, right now, it’s not supported by Zammad that an Agent can also be a customer at this moment. This might work, but the reliable way here is to communicate via Mail with this agent.
We have a pending development for this, but right now still are missing the funds ( see: Feature: Allow agents being customer in other groups they don't have access to ).
As far as I’m aware you currently can’t workaround this - also the two “new ticket” buttons should be one as soon you remove the customer role.