Originally reported on Russian IST board: http://www.phpbbguru.net/community/topic16064-420.html#p326091

If:

1. output_buffering is set to 4096 in php.ini;
2. gzip compression is enabled in phpbb;
3. a warning or a notice is produced somewhere,

then the resulting output is not a valid compressed stream, although it has gzip headers, and results in different browsers either reporting a compression error or just displaying nothing.

—

In includes/functions.php line #3746 (current develop-olympus) there's a check:

				// flush the content, else we get a white page if output buffering is on

				if ((int) @ini_get('output_buffering') === 1 || strtolower(@ini_get('output_buffering')) === 'on')

As per PHP manual on output_buffering setting:

You can enable output buffering for all files by setting this directive to 'On'. If you wish to limit the size of the buffer to a certain size - you can use a maximum number of bytes instead of 'On', as a value for this directive (e.g., output_buffering=4096). As of PHP 4.3.5, this directive is always Off in PHP-CLI.

Thus, the value of output_buffering setting in switched on state can differ from 1 or on.
This causes page can't be displayed on E_NOTICE or E_WARNING in some cases.