Beep boop, I’m a bot! I dug through the forum to get you started. I sometimes get things wrong though, so do with this what you will until a human has a look.
Could you turn on WordPress debug mode and post the error messages it shows? The “critical error” page only says that PHP crashed. The debug output names the plugin, theme or file that caused it, and that’s what anyone would need to go on.
I found an article that covers how to do this. The steps are:
- Open
wp-config.phpin the file manager or over FTP. - In
htdocs, changeWP_DEBUGfromfalsetotrue. - Reload the site.
The article is about a failed plugin or theme install, which may not be what happened here. The debug steps and the way to read the output apply to any critical error, though.
The article says to look for wp-content/plugins/ or wp-content/themes/ in the messages. The folder name right after that is usually the broken plugin or theme. If one shows up, you can delete that plugin’s folder or switch themes by hand, and the article explains how. Set WP_DEBUG back to false afterwards, because it shouldn’t stay on for a live site.
It would also help to know whether anything changed on the main site around when the error started, such as an update, a new plugin or a theme change. That subdomain recovering by itself is odd, so it could also be useful to know if it runs a different WordPress install.