500 error + too many redirects

About two weeks ago I noticed that on my site the subsites /shop and /shop-za wouldn't load their front- or back-ends. I was working on trying to fix this when the whole site errored out with a 500 code.

To backtrack, today I have tried installing WPMUDev Defender, but that wouldn't load its settings pages, so I deleted it and installed WordFence instead. WordFence ran into errors while scanning. I also tried renaming the Plugins folder to no effect. I've searched extensively for solutions to the subsite loading issue but found very few potential solutions.

Before the crash the debug log was giving me this:
[14-Oct-2018 16:28:12 UTC] PHP Notice: Trying to get property of non-object in /wp-includes/class-wp-query.php on line 3740
[14-Oct-2018 16:28:13 UTC] PHP Notice: Trying to get property of non-object in /wp-includes/class-wp-query.php on line 3736
[14-Oct-2018 16:28:13 UTC] PHP Notice: Trying to get property of non-object in /wp-includes/class-wp-query.php on line 3738
[14-Oct-2018 16:28:13 UTC] PHP Notice: Trying to get property of non-object in /wp-includes/class-wp-query.php on line 3740

Currently my entire site is down with a 500 error, and my host says it's an issue in WPMU not on their end.

It's not the .htaccess, and it's not plugins. It's not the browser cache. All the research I've done points to those, so I'm stumped.