SYMPTOMS
Plesk control panel stopps working after the installation of Zend Optimizer.CAUSE
The reason is that Zend sets a path to php.ini file in the registry.HKEY_LOCAL_MACHINE\SOFTWARE\PHP\IniFilePath=C:\WINDOWS
Therefore, all .php scripts on the server start using this setting and use the C:\WINDOWS\php.ini file for PHP configuration.
RESOLUTION
To fix this problem , the following key needs to be removed:HKEY_LOCAL_MACHINE\SOFTWARE\PHP\IniFilePath.
Then all .php scripts will continue using correct .ini files.