My suspicion would be that you may be experiencing an issue due to CORS configuration or the CUCM's self-signed certificate:
- Check the 'Cross-Origin Resource Sharing' configuration on CUCM to ensure that your web application has been configured for access. All aspects of the origin must match, including HTTP vs HTTPS, host name, and port
- The xmlHttpRequest may also be failing if the CUCM has a self-signed certificate, and that certificate has not been imported into whatever cert-store IE11 uses for validation. Typically this is less visible in FF, as you can easily manually add an exception for the CUCM cert early in testing (e.g. while accessing the CUCM admin pages) which masks the problem. Perhaps you can ensure you can access the CUCM admin page (which should require accepting/importing the cert) before testing the xmlHttpRequest
If neither of these topics helps, you might try viewing the request/response details (esp. CORS-related headers) in the browser's developer tools 'Networking' tab, or possibly a request-monitoring tool like Fiddler.