I inherited a client from another developer that used Gantry 5. The site is designed using the Helium template, and the previous developer had added your Page Preloader Atom. It was working fine on their hosting with 1&1 for many years, then all of a sudden around 1 month ago it stopped working. I'm revisiting this now because I just finished updating their site from Joomla 3 to Joomla 4. So this is not a J4 issue as it was already present with J3.
This is a page for Retirees. They have me do an "Oil Change" on their site ever few months to tidy things up and they don't mess with any of the settings. The only thing they do is add calendar events and new articles when necessary. I have updated the particle to latest version but that didn't do anything.
Part of the error message is this:
0 An exception has been thrown during the rendering of a template ("Rendering Atom 'page-preloader' failed on error: An exception has been thrown during the rendering of a template ("DOM Error 76: Unexpected end tag : div while parsing:
You can see the rest here on this staging site which is on my server:
Disabling the Page Preloader Atom allows the site to function.
Looking forward to seeing what you find out.