After upgrading Ubuntu from 18.04 to 20.04, I noticed that my VM .desktop shortcut throws the error message
–startvm is an option for the VirtualBox VM runner (VirtualBoxVM) application, not the VirtualBox Manager.
Before the upgrade, it simply started the virtual machine referenced as parameter value.
It seems that VirtualBox moved the --startvm parameter from the previous VirtualBox executable to VirtualBoxVM. More infos and links can be found in this VirtualBox ticket.
The (easy) solution was to open the .desktop file in an editor, and change the line
Exec=/usr/lib/virtualbox/VirtualBox ....
to
Exec=/usr/lib/virtualbox/VirtualBoxVM ....
