If the issue still persists then try to hit http://<serverurl>/virtualviewer/? (Replace the server url )in the standalone browser window and see if you are able to see a virtual viewer frame. if yes, Let us know so that we can debug further. If not, Please check the virtual viewer server logs once and see if you see any error in server startup.
Just double checking with you, hope you have just added the "/?" to the original url on https as attached in the first document of the issue, so that it reaches the Pega application to retrieve the document.
1) Please try with other type of files like png and let us know if that works. If that works fine, in that case the issue might be the specific docx file you are trying to preview in your previous comment.
2) In case png file is also giving same error. Please make sure that the 2 servers (Virtual viewer server and Pega Server) can communicate with each other. You can try pinging one server from another.
As per the first document uploaded by you, I see that the Pega is running on http but for Virtual viewer you have mentioned https. Just change the VirtualViewerURL DSS to using http and see if error is resolved.