I found the Preferences, Input, Virtual Machine, Host Key Combination setting pretty easily and changed it from the default Right Ctrl to Left Ctrl + Left Shift, considering it's far more rare for me to press two modifiers at the same time, other than Ctrl-Alt perhaps.
No soap. The Right Ctrl stayed as the modifier! I thought maybe I had to try turning it off and on again. Maybe exit VirtualBox entirely? Still nothing. Every time, the preference would reset back to the default.
I even found the configuration file ~/.VirtualBox/VirtualBox.xml on my system, though the new default is ~/.config/VirtualBox/VirtualBox.xml but it was not obvious what I might want to change the value to.
<ExtraDataItem name="GUI/Input/HostKeyCombination" value="65508"/>
Then I found a bug report!
Comment 1 had my answer:
- Open VBox "Preferences..." / Input page / Virtual Machine tab.
- Change the shortcut for "Host Key Combination" in embedded-editor of the shortcut table and (!) press Enter (or Return) key while this shortcut embedded-editor is opened. GUI applying shortcut changes only in that case as this is the default Qt behavior. If you simple press OK button instead your change will be discarded.