Ok. New ISPCONFIG 3 server in place and all websites/accounts migrated. I am not able to access the web management page using the primary domain: http://primarydomain.com:8080 However I am able to access via http://tertiarydomain.com:8080 DNS entries all look ok. Also, I can't access squirrel mail. @ http://primarydomain.com/webmail
You seem to have a dns problem with your primary domain, as access to port 8080 does not depend on the domain name that is used. Compare the IP of the primary domain by pinging it from your local workstation and then pring the tertiary domain as well.
You were correct. After testing on the LAN, I had forgotten to remove the manual entries from my hosts file, and also had the gateway trying to use the local ISPCONFIG as DNS server. Corrected both issues and administration is working fine.