"FastCGI max. Requests" cannot be set to zero (0) in System -> Server Config -> FastCGI tab
Zero (0) is a perfectly valid value for "FastCGI max. Requests" (PHP_FCGI_MAX_REQUESTS environment variable).
As such, ISPConfig should not throw a validation error ("ERROR FastCGI max. Requests is empty.") when the user attempts to set this field's value to zero and click the "Save" button.