
Hi @Zenon Husarewych,
Three conditions can cause this error:
- The web app has reached a billing limit and your site has been disabled.
- The web app has been stopped in the portal.
- The web app has reached a resource quota limit that might apply to a Free or Shared scale service plan.
So, if this is a 3rd party you find in the office store, it means that it is currently unavailable. If this is an add-in developed by yourself, follow the steps in Web Apps: "Error 403 – This web app is stopped".
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