If you have recently updated your SocialEngine to 4.8.6, you may start to see the following error within your errorlog:
Fatal error: Declaration of Engine_Form_Email::isValid() must be compatible with that of Zend_Validate_Interface::isValid()
In file “/application/libraries/Engine/Form/Email.php” (around line number: 73), please search for the below code:
public function isValid($params = array())
And replace the above code with this one:
public function isValid($params)
This solution will resolve the issue on your website.