Visvanath Ratnaweera

Put a test.html file in the Moodle code directory, where the config.php is, and visit http://localhost/test.html.

It that doesn't render the test.html, then your web server is not set up properly. Study its log files. If the test.html shows, then create a phpinfo.php file with content

<?php phpinfo(); ?>

and see whether you see your computer's PHP environment displayed.

I have a language issue with "hidden but not available". The "but" joining the two sounds like "not available" is not one would expect. In my ears the two should be "hidden and not available" or "hidden and unavailable" and "hidden but available" (through the link). Nowadays people are used to "hidden" links, also called unlisted or "hashed" URLs. They are resources which you can open only if you know the URL, sometimes the same URLs you would see otherwise as in the case of Moodle, or the link contains a secret code, which is needed to open it, example "unlisted" YouTube videos.

The text format (*.txt) is quite universal. You might not even need additional text files, the text can be typed straight in to the assignment.

Never came across UptimerRobot. Sounds like a very practical thing. Apparently it monitors many indexes - not only uptime. Now in this graph what does it measure, the percentage of what? Do the peaks there coincide with the connection timeouts you get?

There is no vertical grid to have a careful look, do you say that the peaks have a time pattern, say so many minutes past the hour?

What is the machine, a VPS? a dedicated server? Does it do anything else other than running the four Moodle instances?

P.S. Could be a topic for the Hardware and performance forum in the long run. Pl. don't re-post, the moderator might move the whole discussion.

You are right. https://docs.moodle.org/403/en/Step-by-step_Installation_Guide_for_Ubuntu (Moodle 4.3) has a note "This document is about installing Moodle 4.0 in an Ubuntu 20.04 server with PHP 7.4." and that's it. I'm sorry.