Hi,
Thank you for the details. I think the problem was, that the system was already out of memory. You had 256MB memory limit set, which seemed to be low. I have increased this limit to 512M within the wp-config.php file, via adding these lines:
// Memory limit options
ini_set('memory_limit', '512M');I have also enabled caching to fully minimize the server load during requests: https://i.imgur.com/P0aEPoo.png
I checked your configuration as well, and enabled the index table engine, to make sure all queries are properly cached as well.
Now the performance should be optimal, with the highest performance options.