Which version of QuickBox are you running?
How many active torrents were there when this happened?
How many torrents in total were there when this happened?
More than likely this is a result of php memory limits.
You could try this to see if the problem resolves. I assume you're running on PHP 7
sed -i.bak -e "s/post_max_size = 8M/post_max_size = 64M/" \
-e "s/upload_max_filesize = 2M/upload_max_filesize = 92M/" \
-e "s/expose_php = On/expose_php = Off/" \
-e "s/128M/512M/" \
-e "s/;cgi.fix_pathinfo=1/cgi.fix_pathinfo=0/" \
-e "s/;opcache.enable=0/opcache.enable=1/" \
-e "s/;opcache.memory_consumption=64/opcache.memory_consumption=128/" \
-e "s/;opcache.max_accelerated_files=2000/opcache.max_accelerated_files=4000/" \
-e "s/;opcache.revalidate_freq=2/opcache.revalidate_freq=240/" /etc/php/7.0/fpm/php.ini
# ensure opcache module is activated
phpenmod -v 7.0 opcache
Then do a restart of both apache and php7.0-fpm
service apache2 restart
service php7.0-fpm restart