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If there is an error like "404 not found" and Confluence version is below 5.10 then an upgrade to confluence 5.10+ could help.
Another reason can be Confluence is not accessed via its base url. This could happen if the base url at time the SharePoint Add in was downloaded from the confluence was different from current base url.
Checking login to Confluence
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Error means there was either no registration of the application (see also last Section of Deploy Add-in package to the App Catalog (v1v 1.0) requiring to register the app with the correct Client ID, Confluence host and provider url), communication failed with the app web. Reasons for the later can be:
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