I am having a similar issue.  We are setting this up to move to production. It was running in Dev.  It seemed to stop working for computer devices though it does work for mobile devices.  We are using JAMF Pro in the cloud.  We have been trying to get it running again.  The credentials work, though we tried new credentials anyway. Tried using JAMF classic.  (Though we have JAMF Pro).  I am not sure on which side the error lives - JAMF or ServiceNow.  We are hung up on this right now. 

org.mozilla.javascript.JavaScriptException: Error: Unable to make a connection to JAMF API call. Please check your credential configuration. Check system log for more details. HTTP return status code was 500 (sys_script_include.3fa7da3673b210104ae4b41afaf6a727.script; line 57)