Unable to access Plesk with the error in web browser:
500 Internal Server Error
%plesk_dir%admin\logs\php_error.log the following can be found:
PHP Fatal error: Call to undefined function of_get_key_by_product() in C:\Program Files (x86)\Parallels\Plesk\admin\plib\functions.php on line 3070
Corrupted Plesk installation.
Check Antivirus software quarantine.
Some software protection utilities like Symantec Endpoint Protection treats Plesk's dlls as threat and removes them. Restore those dlls and add exclusion for them.
Otherwise contact Plesk Technical Support in order to resolve the issue.