Thanks for the quick response. I had tried clearing the caches, multiple hosts and browsers + incognito.
I have now found the fix from another post, something got a bit broken on the ADSConfig.conf file during the upgrade, including asking for activation keys again and broke my authURLs however, I had fixed those.
The final fix for me was found here :
failed-to-login-to-remote-instance
Mike @ Cubecoders posted : If you’re using controller/target mode - make sure MapAllPluginStores is turned on in the AMPConfig.conf files of each ADS instance.
I had noticed in my Chrome browser DEV panel’s console tab; was reporting an error loading the MinecraftServer plugin.
Hope this helps someone.