@Georg Pavlovic
Thank you for your post!
Based off my research the Rights Management Adhoc license is a free offering that anyone in an organization can use after they sign up. When a person in your organization signs up for this offering, Microsoft allocates 50,000 seats of RIGHTSMANAGEMENT_ADHOC to your organization. Licenses will be assigned to users only if they sign up. For more info.
RMS for individuals and Azure Information Protection
Related issues:
whats is the use of rights management adhoc license?
Is a rights management adhoc licence / license free?
If you have any other questions, please let me know.
Thank you for your time and patience throughout this issue.
----------
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.