502 error for admin-ajax.php. No ajax functions available

Our company website stopped loading admin-ajax.php today. It gets a 502 error and says “The page you are looking for is currently unavailable. Please try again later.” I cannot load the media library, delete plugins, install plugins, search for plugins, or use any other ajax functionality on the website whatsoever. Our site inventory as well as article filters user ajax as well, so no inventory will load will this issue is going on. Nothing in the file system has changed from when the website was working correctly.

If I disable all plugins and change the theme, the issue persists. Nothing I have tried has resolved this issue and I’ve been at it for many hours. My hosting provider did a file restoration. Admin-ajax worked for 1 minute, and then proceeded to stop working again. The hosting provider is IONOS. They said all looks fine from their end with their servers and offered no suggestions.

I have never seen this issue in all of the years I’ve been building WordPress sites. I am stumped and have not found any information that has helped me resolve this issue. Any help is greatly appreciated.

The page I need help with: [log in to see the link]