The error message:

Call to undefined function checkdnsrr() in ajax_functions.php” file.

Solution:

Check and enable the missing PHP module: http://php.net/manual/en/function.checkdnsrr.php

The best solution is to have that module enable because this function checks emails validity and does not allow suspicious email addresses to be registered.

The 2nd solution is to delete the code that asks for this module in the code.