Internal server error admin-ajax.php

We cannot test emails due to internal server error 500 on admin-ajax.php. When we click the envelope button to send a test email, nothing happens, and the browser console displays the 500 error. We’ve spoken with our hosting providers, and they have been unable to see why this is happening. Logs show this error: AH01071: Got error ‘PHP message: PHP Fatal error: Uncaught Error: Call to a member function getOffsetTimestamp() on null