Moderator Tim Post closed my question "VirtualBox upgrade trashed my virtual machine" as off topic. I have no problem with this. The problem is that he closed it just one hour before my 500-point bounty expired. So I got my 500 points back, and the user who deserved the bounty never got it! Please somebody, fix this injustice if you can.
Edited to add: OK, I've fixed it myself.