BIP KB:
Proxmox NoVNC not working

Well, if you are having problem with PROXMOX NoVNC not working and has been ignoring it up until now, its time to make it work. NoVNC basically uses web socket and html5 to allow you to remote access your virtual machine. So make sure you use a browser such as Chrome instead of Safari which has a full compatibility of web socket implementation on the browser. If not, you will most likely get yourself an error such as this.

TASK ERROR: command '/bin/nc -l -p 5900 -w 10 -c '/usr/sbin/qm vncproxy 100 2>/dev/null'' failed: exit code 1

Due to compatibility issue, Proxmox NoVNC might not work with the default install. All you need to do is to find out which NoVNC works for your current Proxmox installation and down/upgrade it! And for me, the version NoVNC 0.47 works for me so i downgraded it from 0.53 by doing the following,

wget http://download1.proxmox.com/debian/dists/wheezy/pvetest/binary-amd64/novnc-pve_0.4-7_amd64.deb
dpkg -i novnc-pve_0.4-7_amd64.deb

And it will do the rest, and if you would like try other version, just head down to the following link

http://download1.proxmox.com/debian/dists/wheezy/pvetest/binary-amd64/