WordPress site is down with a 500 or 503 error in Plesk

Symptoms

Some of these errors suddenly started affecting a WordPress website hosted in Plesk (after updates, migration, or even if no changes were made):

• The site, or parts of it, won't load, with errors in the browser or dev console

  500 external Server Error

  503 Service Temporarily Unavailable

• The admin dashboard shows this message

  There has been a critical error on your website. Please check your site admin email inbox for instructions

• Disabling all plugins brings the website back up

Cause

Coding issue on the WordPress instance caused by malfunctioning or incompatible plugins, themes and/or customizations.

Resolution

Find out which plugin is causing the issue and disable it.

1. Log in to Plesk
2. Disable all plugins in WP Toolkit > example.com > Plugins tab
3. Enable plugins one by one, checking the site after each one
4. When the problematic plugin is identified, disable it and enable the rest

A web developer should review the plugin to find a way to make it work. Possible solutions include reinstalling or updating the plugin, and changing to a different PHP version in case it is not compatible with the current one.