I migrated a Redmine installation from an Ubuntu machine to a Debian one (both 32-bits), and now for some reason, for some users it doesn't return the page but only a 200 OK message. Here is the flow (from Wireshark):

GET /issues/142 HTTP/1.1
Host: debian:3000
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:17.0) Gecko/20100101 Firefox/17.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Cookie: _redmine_session=BAh7DCIQX2NzcmZfdG9rZW4iMStIM1RBNTlNelZVUXlUazgrR1pUNGUvNGdEbytUZzRyMVFSUnBvNGhlSDg9Ihd0aW1lbG9nX2luZGV4X3NvcnQiEnNwZW50X29uOmRlc2MiD3Nlc3Npb25faWQiJThiMDk0MzVhOTEzYTI0MzVjOGEzYTRmNDU0NzcwMTAwIgx1c2VyX2lkaQoiFmlzc3Vlc19pbmRleF9zb3J0IgxpZDpkZXNjIg1wZXJfcGFnZWlpIgpxdWVyeXsHOg9wcm9qZWN0X2lkaQc6B2lkaQo%3D--8588c221c0642a12f396239455fb702aec14c9c9; my_wiki_session=f70ae11e1c533c86f0e039d63cf3f69c; my_wikiUserID=1; my_wikiUserName=Yasin
Cache-Control: max-age=0

HTTP/1.1 200 OK 
Connection: Keep-Alive
Date: Wed, 12 Dec 2012 16:30:16 GMT
Server: WEBrick/1.3.1 (Ruby/1.8.7/2010-08-16)
Content-Length: 0

As you can see, I get nothing from the server. This is mostly random because this blank page happens sometimes for some users, and for other users it almost never returns the page... I'm absolutely lost here.

Any idea about what can be the cause? Thanks in advance!