One of our clients has logged a very strange issue with us- We launched a preview for their website, but when it's viewed on their main PC, peculiar things start to happen...

At first, the stylesheet wasn't being found, and so accessing any page resulted in one void of all styles. We sent them a direct link to the stylesheet, which was viewable from all our computers in the office - but gave a "File Not Found" error on their side. I then deleted the file, and replaced it with a new blank file, which he could then access. Copy-pasted screen.css contents into this file, and he could then view it fine, and stylesheets magically worked on the site again.

Now, he can view styles, but not the referenced header images. The strange thing is that this problem doesn't exist on any other PC we've tested, or on any other site on the problem computer, but obviously we'd like our client's site to work for them. The strange thing is, they can view other sites of ours, hosted on the same server, built on top of the same CMS (and so most of the files are the same) without problem - but are getting 404s for files that most definitely do exist.

• Stylesheets are not turned off, nor is anything specifically deactivated on their browser (as other sites are fine)
• Reloading with CTRL+F5 doesn't help
• The client is using the latest version of firefox

Any ideas here on what to try / how to narrow the problem down?