I though of another thing to try. Make sure you have this in the head section of all templates (with the exclamation point).
<base href="[[!++site_url]]" />
I'd clear the cache files again after doing that.
If that doesn't do it, it could be that your host has upgraded PHP and your version doesn't work well with it. Since your site isn't working, I'd say you've got nothing to lose by upgrading MODX and your extras. As you upgrade to newer versions of MODX, more available Extra updates will start showing up in Package Manager.
Use this method to upgrade: https://bobsguides.com/modx-upgrade-faq.html
Be sure you have at least PHP 5.4 running.
Once you hit MODX 2.4, you should be able to upgrade to PHP 7 (assuming that you've fixed the bug), then uninstall and remove Upgrade MODX, re-install it, and use the UpgradeMODX Extra for the rest of your upgrades. For security, and to prevent future hassles, keep your MODX install up to date.