One of my app customers is getting "Failed to create Confluence page. Status: 403. Response: {"code":403,"message":"The app is not installed on this instance"}"

Forge
, ,
1

I have a Forge Confluence App that talks to my custom BE server. My server is performing an API POST request to /wiki/api/v2/pages:

let response = await fetch(`${apiBaseUrl}/wiki/api/v2/pages`, {
      method: "POST",
      headers: {
        Accept: "application/json",
        "Content-Type": "application/json",
        Authorization: `Bearer ${userAuthToken}`,
      },
      body: JSON.stringify(bodyData),
    });

This code works for all my customers, except for one. That specific customer is getting an error from the Confluence API:
Response: {"code":403,"message":"The app is not installed on this instance"}

Please help!