Hi,
Thanks for posting your question in Microsoft Q&A forum.
Based on your description, the error suggests a failure in the Rights Management System (RMS) when attempting to process the tenant's encryption requirements.
Have you verified that whether Azure Rights Management is active? If not, then you can manually activate Azure Rights Management. You can also verify Microsoft Purview Message Encryption configuration in Exchange Online PowerShell. Just follow the steps in this link: Set up Microsoft Purview Message Encryption | Microsoft Learn.
Please feel free to contact me for any updates.