dns hosting – Possible dns issue after Tomcat 8.5 update, buttons not working only on dns url

Recently we had an update from tomcat 6 to tomcat 8.5.

When I am trying to login to my web gui via direct link it is working fine but when I used the dns url, for first time it works fine but if we create another window with same dns link, some buttons in the jsp are not working. I checked the application logs but there were no logs for the actions that should occur after click. And I checked tomcat catalina.out and localhost logs but there is no errors there, but when I saw the localhost access logs I found that I was getting 304 issue, so I cleared the cache and closed all browser. And then I opened the dns link again, for the first time I got to the login screen and not having this issue but I am having the same issue when opening a second window with dns url where the login screen is skipped as the previous session is still running.

I am not facing this issue with direct urls but just on the dns url, so I was thinking it is not an application issue.

Can you please help me, if this is a tomcat issue or an issue with dns and where I should look for next