Apache Basic Auth not working in customer network

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http://serverfault.com – I'm having trouble, letting a customer see a site that is locked with a simple Apache Basic Auth password file. It works in every browser on Mac, Windows and in our network. Even the customer get's it working at home, but not in his office. What could possibly go wrong here? Some Proxy caching something? UPDATE: My config that has always worked in projects either in .htaccess or vhost config AuthType Basic AuthName "Customer - Project" AuthUserFile /mnt/drbd0/www/staging.hostname.com/site/include/passwords Require user staging.project UPDATE2: And here's the relevant part from the error (HowTos)