Hi @MelleBoudewijns-1749
Thank you for reaching out!
I am unable to reproduce this behavior. I'm able to create and further update birthday property of a contact record successfully.
Generally 406 error signifies "This service doesn’t support the format requested in the Accept header". Please ensure that you are using right content-type. i.e., For PATCH request, you need to add Accept application/json in header.
Refer- https://learn.microsoft.com/en-us/graph/errors#http-status-codes
Hope this helps.
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