Mailman Web pages not showing all in Virtual URLs

I have a The Perfect Server - Ubuntu 18.04 (Bionic Beaver) with Apache, PHP, MySQL, PureFTPD, BIND, Postfix, Dovecot and ISPConfig 3.1 that works fine, except for one small issue - Mailman only shows its webpage on two of the virtual URLs and not all three URLs.
I have six domains in vhosts on the server, each with working Apache2 and Postfix implementations. I have about seven Mailman lists across three of the domains and mail is sent to all seven of the lists as expected. Each list is configured with their own URL, but two of the lists that are referenced on the third domain do not display their web pages. Apache gives a 404 when domain3.tld/cgi-bin/mailman/admin/ is referenced, but the same syntax on domain1.tld and domain2.tld shows valid Mailman pages.
The forum won't let me post links, so I cannot show the actual pages. I have not made any changes to mm_cfg.py or mailman.conf, per the Perfect Server instructions. Two domains work, but the third domain does not.
I posted a much longer explanation of my problem, but it was lost in a closed tab, so I hope my explanation of the situation makes sense.
Where should I look? Thanks.
Gibson