Most times it’s very useful to have a look at the webserver logfile, the own logfile of ownCloud or the logfile of php. On linux you can find (depending on your used distro) at this locations:
ownCloud (access on the file system directly->Login via ssh-terminal/ftp, NOT via browser):
/path/to/owncloud/data/owncloud.log
IMPORTANT NOTE:
Please set DEBUG to true (for 8.1 and below):
or debug => true (for oC 8.2+):
and the loglevel to 0:
in your config.php (just add the two directives above) to get more logging output.
Apache
/var/log/apache2/error.log
/var/log/apache2/access.log
NginX
/var/log/nginx/error.log
/var/log/nginx/access.log
lighttpd
/var/log/lighttpd/error.log
/var/log/lighttpd/access.log
Webhosting
Can be located in a separate folder in your home-directory or in the managing-interface of your provider. In doubt, check your webhoster’s FAQ and manual or ask their support.
PHP
Source of this info: http://davidwinter.me/articles/2009/07/14/enable-php-error-logging/
The plain PHP installations shows error instead of logging them. To enable the error log of php you have to edit your php.ini (e.g. /etc/php5/apache2/php.ini) and set these options:
display_errors = off
log_errors = on
error_log = /var/log/php-errors.log
Additional you have to create this logfile with the proper permissions:
sudo touch /var/log/php-errors.log
sudo chown www-data:www-data /var/log/php-errors.log
After this edits you have to restart your webserver and/or your php fcgi process.