Apache gives empty reply

Solution 1:

Look at the main server error log, it should have some Child Segfault Error (11).

This is probably given by a PHP engine fault, often given by a buggy php module, unicode chars handling, gettext corrupted DB, and so on.

It is very difficult to debug.

You can enable xdebug tracing and look at what line it is segfaulting:

pecl install xdebug

php.ini:

[xdebug]
xdebug.profiler_enable=1
xdebug.profiler_output_dir="/tmp/xdebug"
xdebug.auto_trace=1            ; enable tracing
xdebug.trace_format=0
xdebug.show_mem_delta=1        ; memory difference
xdebug.show_local_vars=1
xdebug.max_nesting_level=100

Solution 2:

Finally solved by adding to /etc/apache2/envvars:

export USE_ZEND_ALLOC=0