On OES 2018 SP1, the IPP page fails to display, if the iprint_ssl.conf and iprint_g.conf files are customized on OES 2015 SP1 server. Upgrading to OES 2018 SP1 changes both these (iprint_ssl.conf and iprint_g.conf) files to accommodate requirements of the new version of Apache (Apache 2.2 to Apache 2.4).
To resolve this issue, refer to the TID 7023593. The steps in the TID are also valid for OES 2018 SP1 server.