Teams incoming webhook replying with status 200 but with message "Microsoft Teams endpoint returned HTTP error 403"

Question

Hi there,

Since March existing incoming webhook to Microsoft Teams has stopped working. Sending a test message with cURL fails as follows:

curl -H "Content-Type: application/json" -d "{'text': 'Hello World'}" https://my-company.webhook.office.com/webhookb2/<hidden-value>@<hidden-value>/IncomingWebhook/<hidden-value>/<hidden-value>/<hidden-value>

Webhook message delivery failed with error: Microsoft Teams endpoint returned HTTP error 403 with ContextId tcid=0,server=msgapi-production-cse-06-0,cv=bPrwEI67gEaB3taZVjz9TQ.001=2..%                 

Even creating new Incoming Webhooks fails with the same message. On "Teams Admin Center" I see that "Incoming Webhook" is Available to everyone, App status in "unblocked".

Anyone has any tangible hint on where I could check for wrong permissions? I'm full administrator so should be able to check and change everything.

Thank you all so much!