Unable to access Plesk: Warning: Unknown: failed to open stream: No such file or directory in Unknown on line 0

Symptoms

Unable to access Plesk:

PLESK_INFO: Warning: Unknown: failed to open stream: No such file or directory in Unknown on line 0
Fatal error: Unknown: Failed opening required 'auth.php' (include_path='.;C:\php\pear') in Unknown on line 0

or

PLESK_INFO: Warning: Unknown: failed to open stream: No such file or directory in Unknown on line 0
Fatal error: Unknown: Failed opening required 'auth.php' (include_path='.:') in Unknown on line 0

The following command didn't resolve the issue:

C:\> "%plesk_bin%"\ai.exe --select-product-id panel --select-release-current --reinstall-patch --install-component base

Cause

Failed upgrade, not all packages were upgraded.

The provided command is not applicable to Plesk Onyx.

Resolution

For Windows:

1. Connect to the server via RDP;

2. Complete Plesk upgrade:

   On Windows

   C:\> "%plesk_bin%\\ai.exe" --console

   Or

   C:\> "%plesk_bin%"\ai.exe --select-release-current --reinstall-patch --upgrade-installed-components

3. Additionally make sure that file `C:\Program Files (x86)\Plesk\admin\conf\php.ini` is not empty.

For Linux: 

1. Connect to the server via SSH
2. Execute the following:

   # plesk installer update --repatch