What is necessary for a virtualbox machine to be used with different hosts (linux and windows)?

One big advantage of Virtual Machines is the fact that they are somewhat independent of the underlying hardware. My goal is to have a Virtualbox image on an external USB-drive that I can use with different host systems (some of them are Windows 8.1, some Debian Linux).

In the past this has worked fine, but with the latest releases of Virtualbox the Windows host refuses to start an image created on the Linux host because of the "already existing" image for the VirtualBox Guest Additions. (Of course the path of them is different in Windows from the one in Linux).

So the question is: What configuration settings are necessary for an image to be usable across several host systems?

It is clear that the image has to be stored on a filesystem that can be read and written by all host systems, so this is NOT the scope of my question.

exact Error text:

Konnte die virtuelle Maschine in E:/virtualmachines/Wheezy-Test1/Wheezy-Test1.vbox nicht öffnen.

Cannot register the DVD image 'C:\usr\share\virtualbox\VBoxGuestAdditions.iso' {7fbf3153-636d-4496-9bea-da60ebb6f8ba} because a CD/DVD image 'C:\usr\share\virtualbox\VBoxGuestAdditions.iso' with UUID {ac809a6e-292f-4cb4-81c4-bc57caa2f595} already exists.

Fehlercode:E_INVALIDARG (0x80070057)
Komponente:VirtualBox
Interface:IVirtualBox {fafa4e17-1ee2-4905-a10e-fe7c18bf5554}

It is more like a workaround than a real solution:

Since the dismounting of the GuestAdditions via the User Interface didn't work (see my comment above) I had to edit the .vbox file and changed the <DVDImages> ... </DVDImages> element to an empty <DVDImages/>. No I still get the warning that there is no GuestAdditions-Image available but at least the VM can be started now. (And adding the GuestAdditions should be no problem if needed)