Since friday we've been unable to install our SharePoint App (AddIns). The installation fails with a message "The remote event receiver callout failed". This happened on multiple tenant, including those where we previously could install the app.
Unon investigating it appears that Microsoft has made a change in the remote event receiver request, causing TokenHelper.cs to fail because the "intended audience" is not valid. This is because the request has changed format of the url, adding azurewebsites.net to it (or whatever host you are using).
I found the following fix until we know more:
Add an application setting called "HostedAppHostNameOverride".
If your app is located at myapp.azurewebsites.net then set the value to "myapp;myapp.azurewebsites.net".