Cause for webserver problems found
After two weeks without problems, I’m now confident that I have found and fixed the cause for the webserver problems on 25 and 26 September.
Two security plugins for WordPress were the offenders, “Acunetix WP Security” and “Ultimate Security Checker”. After the last update, the first one began to carelessly make many long-running database queries. Meanwhile, the second one had inflated WordPress’ crucial wp_options table over the course of time. I guess a webserver which is no longer online after a database crash is kind of secure…
This shows that some functionality that might be beneficial for a website with 1000 page views per day can be disastrous for a website with 100,000. Apart from deleting the two plugins, I worked on the website backend during the last two weeks to further increase its performance.