A website throws following pop-up messages on every singe page:
Some files on the server may be missing or incorrect. Clear browser cache and try again. If the problem persists please contacts website author.
After a quick look into website's PHP code it became clear where the message was coming from. The website was developed using Adobe Muse. Every page had a little script added my Adobe Muse that throws the above error if its algorithm determines that file versions are not sync.
To resolve the issue and avoid this in the future you need to upload modified website files directly from Adobe Muse. If that's not possible and you have to export the project first, upload all files and not only those that you changed.