@Maximilian Kirschning Thanks for reaching out. I have looked into backend logs for the reported ID: b016d2d8-b629-4d56-bec6-4cbf82f23032 and I can see that the event grid subscription failed due to your destination endpoint i.e. azure function (in your case) being in a
disabled state. Please make sure that you have enabled the function app and retry the operation again i.e. create the event grid subscription on your storage account. Please refer to my private comment for more details on the function app.
Feel free to get back to me if you still face the issue once you have enabled your function app.
The feedback is already passed to the product team to make these errors user-friendly rather than showing the internal error.