If you are updating from v2.3.7 to the current v2.4.2, I would suggest you check out this post.
You will need to perform just a couple of manual tasks to meet the needs of the new directory structure of your /torrents directory.
3200 torrents loaded could produce the error in regards to the php.ini settings. You can up the memory limit by doing the following:
sed -i "s/memory_limit = 768M/memory_limit = 1024M/" /etc/php/7.0/apache2/php.ini
service apache2 restart
Although 768M should be a high enough value, sometimes having several thousand could require a bump.
Also, just double check that your space requirements are in order. ruTorrent will go into kill mode if there is not enough disk space. Once you update to v2.4.2 you can also run a couple of other commands to ensure a clean system.
removepackage-cron- this will set you up-to-date with the new systemd that has been implemented.