We are using a custom error display by subscribing to the 'error.groupdocs' event. During testing, we've found that the event isn't raised if the server is unavailable.
E.g. the document is initially loaded OK, but then the server goes offline, then either a new document is attempted to load (using loadDocument method), or the user scrolls to a new page that isn't preloaded on the client. In this case, the "Loading your content" message appears and doesn't go away, and the 'error' event is not raised.
If think the problem is because of this line in the _errorHandler function in groupdocsviewer.all.min.js:
if (b.xmlHttpRequest.readyState == 0) return;
The readyState will be 0 if the request couldn't be sent because the server is offline. In Chrome there will be a line in the console: 'net::ERR_CONNECTION_REFUSED'