Hello @Andries den Haan
Thank you for sharing the details.
I tested this behavior in my test environment and was able to reproduce the issue when attempting to add a mapping from a managed property. Based on the error messages shown in your screenshots, the issue appears to be related to an internal server-side problem within the Search Schema service.
However, when I tested mapping a crawled property to a managed property, I was unable to reproduce the same behavior. As a result, could you please try mapping a different crawled property, preferably one that starts with ows_?
If the issue persists across multiple crawled properties, I would highly recommend contacting Microsoft Support directly. Since the error appears to be occurring on the backend, Microsoft Support can investigate whether there is an active service issue or escalate the case to the appropriate engineering team for further analysis and remediation.
To create a support ticket, a Microsoft 365 administrator can follow the steps in this article: Get support - Microsoft 365 admin | Microsoft Learn
If you are not a Microsoft 365 administrator, please contact your administrator and ask them to create the support request on your behalf.
I hope this help. If you have any further concern, please feel free to let me know.