So, this is a very stupid issue, and a really annoying one as well :) . But really simple to fix. These instructions are valid for apache 2.2.22, with default settings

Diagnosis

run following command

tail /var/log/apache2/error.log

If you see something like following

[Wed Feb 20 19:31:19 2013] [error] [client 127.0.0.1] PHP Warning: Cannot modify header information - headers already sent by (output started at /var/www/vtiger/include/utils/ListViewUtils.php:1) in /var/www/vtiger/index.php on line 91

“/var/www/vtiger/include/utils/ListViewUtils.php” could be any file path, similarly “/var/www/vtiger/index.php” could be any path as well. The key over here is the :1. That means the issue is at the line 1. This might be because of something called Byte Order Mark or BOM.

Fix

Open “/var/www/vtiger/include/utils/ListViewUtils.php” in vim
and save the file without a BOM:

:setlocal nobomb :w

To ask Vim if there is a BOM:

:setlocal bomb?

Simple, this should fix it.