Microsoft office web apps server 2013 can’t be installed on the same machine as a Microsoft SharePoint server product
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Cause: The server you choose can't have SharePoint running on it or any web application running on port 80. Also, Microsoft Office (or any Office component) cannot be installed.
Solution: There is no workaround or solution for this apart from choosing a separate server to install web components. We must choose another server for Web App components.
Servers that run Office Web Apps Server Preview must not run any other server application below:
- Exchange Server
- SharePoint Server
- Lync Server
- SQL Server
No version of the desktop Office suite
See more at: http://expertsharepoint.blogspot.de/2013/12/microsoft-office-web-apps-server-2013.html
Note: If you find a different solution, please report it as a comment to this post. This will be useful to the SharePoint Community. Be sure to double-verify it: undo your solution and verify that the problem comes back, then redo it and verify that the problem goes away.