Hi @Thomas Kison,
Thanks for reaching out.
Error 500 is an expected behavior. As per the documentation, Graph API:
GET /solutions/bookingBusinesses will work only with delegated permission.
Regarding your 2nd issue: HTTP 403 - Forbidden, I would recommend you to raise a support case with Microsoft Graph, a Support Engineer will be able to assist you better. You can raise support ticket from https://aad.portal.azure.com/ or https://admin.microsoft.com/#/support/requests.
Hope this helps.
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