When setting a new second AdminUI behind an already existing and protecting AdminUI Reverse Proxy, after authentication, the new AdminUI doesn't seem to recognize the SMSESSION cookie, and as such, the admin user needs to log in manually in the out of the box AdminUI Form Authentication Scheme.
There are limitations when setting Web Agent on Reverse Proxy to protect AdminUI.
If setting a second AdminUI protected by Apache Reverse Proxy Web Agent, then the Web Agent should run on a distinct and separate Apache Reverse Proxy instance (1).
This implies a separate and distinct Web Agent (2).
Install and configure separate Web Agent and Apache Reverse Proxy to protect the second AdminUI to solve the issue.