THANKS
First I want to thank all of the people who made DietPi possible. It is an amazing solution : all is simple, integrated, easy to understand and everything works as dreamed ! Amazing ! So much amazing I already decided to make a donation. It's the 2nd time in my life I am making a donation for a free software (shame on me not to have made that more often !) so it proves at which point I am greatly positively surprised and loving DietPi

MY PROBLEM
I'm trying to install Plex server on a RaspBerry Pi 3. Install itself is easy and I had no problem with it. Only thing to be careful is to select "en_US.UTF8" as default locale at the end of the install process. Did all of that and it ends saying everything is done. But when I go to http://192.168.....:32400/web my browser says it can't reach the destination. And that's my problem here because it means I can't do the initial first launch setup of the Plex Server.
WHAT I TRIED
I know the Pi is working correctly because I can reach http://192.168... and also http://192.168..../nextcloud works perfectly. I confirmed "plexmediaserver" is running on the Pi.
I tried to use a different browser, I tried to clear the history/cache of my browser, tried to purge the plexmediaserver (following same type of issue found on this forum), even tried to uninstall "plexmediaserver" and then install it but the same situation persists.
MY SETUP
Installed on the DietPi I have dropbear, proftp, lighttpd (mysql), nextcloud, NoIP, PiHole and Plex obviously. So nothing too "weird" and for now I did nothing "unusual". I just followed instructions for installations of each ones and everything works excepting the little details I keep for later resolution (my isp blocks port 80 so I have to found a way to connect to Nextcloud from outside, I have to understand how to have an ssl certificate with NoIP).
ANY IDEA ?
If someone would have an idea how to fix the situation with Plex I would greatly appreciate

If you need more infos from my setup (install logs, etc...) I most probably would need some instructions how to find them as I don't have the pretension to master the command line nor the linux system (yet).
Thanks !