Groups conversation webhook does not trigger

Question

I'm developing a solution in which I want to react on incoming mails from a group mailbox (not a shared mailbox). I have a webhook subscribing the group mailbox and triggers an Azure Function.

In the Azure Function log, I see the registration call with a validation token. However, when I send emails to the group mailbox, the webhook is no longer triggered, and my azure function is not called.

Until last Thursday, my function was still being invoked.

I’m registering the Azure Function with the following body and the endpoint. https://graph.microsoft.com/v1.0/subscriptions

{
    "changeType": "created,updated",
    "clientState": "E-Mail Received",
    "expirationDateTime": "2024-03-13T04:26:28.7712196Z",
    "notificationUrl": "MYFUNCTIONURL",
    "resource": "groups/{GROUPID}/conversations"
}

The registration call was successfull and i can see the subsription by calling the subscriptions endpoint of the graph API.

In the log i can see the registration call. But when i send emails to the group mailbox, the webhook does not trigger.

Can anyone provide a solution for this problem?