404 redirects "stolen" by provider

Hello, I am running a website under apache and php where I do redirects on 404 errors: apache conf: ErrorDocument 404 /subapp_members/search_user.php This is done to allow ULRs with usernames like this: www.server.com/username The PHP script search_user.php looks in a db if the user name is existent and if yes shows his member page. If the name is not existent it displays an internal 404 message. This worked perfectly for recent years until now. Some users complain that they do see advertisement instead of the page. A research showed that they are using a provider called "unitymedia". As soon as a site has a page not found error it redirects them to their own advertisement page. This is true for all pages on the net. e.b. ebay.com/testing shows their advertisement. Has somebody an idea on how to fix that from my site? Thank you for any help, Merlin