Problems with Azure Mobile Services (getting 401, invalid signature)

Hey
Suddenly (or in the last few weeks, just back from vacation) I’m getting

{
     "code": 401,
     "error": "Error: The authentication token has an invalid signature."
}

when calling my webservice hosted at azure mobile services (migrated to app services)
https://[myservice].azure-mobile.net/tables/[MyTable]

Any idea what could have happened?
The token validates on the https://jwt.io/ website.