"Connection error - server offline?" in Web GUI

is you nodename/ip correct in /etc/hosts /etc/hostname ?
do you have any browser extensions installed that might interfere?
 
ip/hosts/hostname - all according to manual.

If I install PVE from iso on same PC everything works fine, so its not browser problem.
 
can you show the content of /etc/hostname /etc/hosts and /etc/network/interfaces ?