I've picked up looking at an issue with a live Moodle site that was built by someone else. Its currently on 2.9.2 and runs on Windows under Apache 2.4.37 and PHP 5.6.40. The Environment check reports all OK.
The issues originally came to light when we tried to move it to use https. Having followed the instructions on implementing https (changed config.php and ran a database search to update urls to https) the site only loads text with no images or CSS. This led me to discover that actually, even without moving to https, if I just click Purge All Caches on the current functioning site, that causes all images and CSS to stop loading permanently and they can only be brought back by rolling back the server from a backup. So basically I have a site where if the cache gets cleared, all CSS and images stop loading. If it gets updated to a newer version, the the CSS and images stop loading. If moved to a different server with same URL, the CSS and images stop loading. Switching to the default theme does not fix the issue.
I found a similar post where it was suggested to enable zlib.output_compression in php.ini which gets the CSS to start loading but without images, and with this turned on all files downloaded from Moodle are corrupt eg Powerpoint files. It feels like the issue must be a config setting in PHP or Apache but I'm at a loss as to what it could be. Any ideas welcome!