Plesk interface is not working after Plesk upgrade: “TypeError: Cannot read property” or “404 Page Not Found”

Symptoms

After upgrading Plesk, some pages fail to load with one of the following error messages:

404 Page Not Found

TypeError: Cannot read property querySelector of null

TypeError: Cannot read property 'map' of undefined.

Cause

The upgrade process was interrupted, and Plesk was upgraded partially.

Resolution

1. Connect to the Plesk server via SSH (Linux) / RDP (Windows Server).

2. On Windows Server, start a command prompt as an Administrator.

3. Start Plesk Installer to complete the upgrade:

   # plesk installer update