Proxmox VE is a complete open source virtualization management solution for servers. The enterprise-class features and the intuitive web interface are designed to help you increase the use of your existing resources and reduce hardware cost and administrating time - in business as well as home use.
ProxMox is a fantastic free virtualization tool, however there a couple quirks to getting it set up properly, one of these issues is the ubuntu plugin error you'll receive when trying to access the web administration console for the first time.
Trying to gain console access using the VNC viewer you may get an unhelpful console error page.
The console viewer makes use of openjdk icedtea project, Proxmox VE Java console does not work with old icedtea-plugins on Ubuntu. So you will need to update your current OpenJDK to version 7.
If you have latest Ubuntu 14.04 LTS, you may just need to install the OpenJDK using the following command
sudo apt-get install openjdk-7-jre icedtea-7-plugin
If your are using an older version of ubuntu you may need to delete your current version of OpenJDK and install the new version
$ sudo apt-get purge openjdk* $ sudo add-apt-repository ppa:webupd8team/java $ sudo apt-get update $ sudo apt-get install oracle-java7-installer
After doing this you should be able to run the Console application using the web browser no problem.
Update : I also found this resource on the proxmox wiki pages that provides a full explanation and resolution. java console (ubuntu)