Plesk domains are not accessible with a 502 error: More than one MPM loaded

Symptoms

• On a Linux Plesk server, domains are showing a 502 Bad Gateway error when opened via a browser
• Apache is unable to start and its configuration check via the apachectl -t command produces the following error:

  AH00534: ... Configuration error: More than one MPM loaded.

Cause

Misconfiguration of Apache has made it attempt to load more than one MPM module, however by design Apache is able to load only one MPM module at a time.

Resolution

1. Log into Plesk
2. Go to Tools & Settings > Apache Web Server
3. Press Apply without making any changes
4. Start Apache at Tools & Settings > Services Management > Web Server (Apache).