Just for completeness sake, here is a - probably better - workaround from the mentioned ticket:
I observed this as well. I had been using VB on two physical systems but wanted to be able to transport my virtual machines from one system to the other. So, I moved the .vdi files to a portable hard drive and changed the Default Hard Disk Folder and Default Machine Folder in Preferences on both systems to point to the appropriated folders on the external drive. I then created new virtual machines on each system and attached the .vdi to them. Then I modified the settings in each virtual machine and set the Snapshot Folder to the appropriate folder on the external drive. This worked well, except that I noticed my saved machine states were getting lost after closing VirtualBox. After some investigating, I noticed in the VirtualBox.xml file in the local .VirtualBox folder that the src= attribute in the MachineEntry? node was pointing to the local path, not the one on the portable drive. Once I modified the src= path to point to the machine's .xml file on the portable drive, VirtualBox was able to load my saved states successfully. Hopefully, this information will help track down and fix the bug.
Once again, thanks for your help throughout this, much appreciated!
Topic is marked solved...