Grav website down (HTTP 500) - can't troubleshoot solution

So, whilst I didn't find the solution myself, nor fully identify the problem in my first post, I can now offer some more information.

It turns out that the SSL certificates were not the problem at all and that installation had been fine. Instead the problem had been with the scheduler. It turns out there was a cron job running which f'd up the permissions.

I pasted this into the terminal and it didn't seem to work and 'no cron available' appeared at the top of the admin panel in the tools section.

(crontab -l; echo "* * * * * cd /home/grav/www/html;/usr/bin/php bin/grav scheduler 1>> /dev/null 2>&1") | crontab -

I thought nothing of it until the website went down. Then with my friends help we got it back online. Until it went down again during the night. At that point, it became obvious it was a permissions issue caused by a cron job.

I believe this is because I ran the code from the terminal logged in as root, instead of logged in as the 'grav' user that I created during the installation of grav.

In the end, I believe my friend opted for the script option on ths page https://learn.getgrav.org/16/troubleshooting/permissions. I don't think it has cured the underlying problem but the website does now remain online at least.

So it's a half solution.