Invalid argument: core_output_filter: writing data to the network
Apache uses the sendfile syscall on platforms where it is available in order to speed sending of responses. Unfortunately, on some systems, Apache will detect the presence of sendfile at compile time, even when it does not work properly. This happens most frequently when using network or other non-standard filesystems.
Symptoms of this problem include the above message in the error log and zero-length responses for non-zero-sized files. The problem generally occurs only for static files, since dynamic content does not usually make use of sendfile.
To fix this problem, simply use the EnableSendfile directive to disable sendfile for all or part of your server. Also see the EnableMMAP directive, which can help with similar problems.