As almost everytime after upgrading Virtualbox to a major version I had trouble to get it up and running on my Ubuntu host. This time I upgraded from 3.2 to 4.0.4 and had the following issues:
Vboxtool no longer working:
I am using Vboxtool to manage my virtual machines from the command line. This tool stopped working because a major redesign of the VBoxManage command. I found a hacked version here. This version works for me.
No access to the virtual machines via RDP:
I was able to start my virtual machines but I could no longer connect to them via the RDP protocol. There were two issues here:
First I neeeded to start the VirtualBox Manager and go to "File -> Preferences -> General". Choose "Reset" in the field "RDP Authentication Library". Afterwards it should read "VBoxAuth" instead of "VRDPAuth" which was here before.
In addition you need to install the Oracle VirtualBox Extension Pack as this seems to provide the RDP functionality. You can download it from here. You need to be root to install it (or run Virtualbox Manager with sudo).
After completeing the steps above Virtualbox now seems to run again ...
BTW: A step by step instruction to install Virtualbox 4.0.4 on an Ubuntu host can be found here.