Hello Egon!
I had the same problem and there is already a bug report.
I believe that you have mounted shared folders?
Try it with the 5.24 version from virtualbox-guest-additions-iso
I extract it from the debian package and since then it works again
> I am running Debian Testing and since recently (1-3 weeks?) an update of
> VirtualBox (I think the version that restored the "Start/Resume" buttons
> that got missing some months ago), restoring saved states for Linux guests
> started failing with this error (the first line is in Dutch, and the
> translation is below the error message), for example here for a VM with the
> name "My VM":
>
> ====================================================================================
> Het is niet gelukt een sessie te openen voor virtuele machine My VM.
>
> The VM session was aborted.
>
> Resultaatcode:NS_ERROR_FAILURE (0x80004005)
> Component:SessionMachine
> Interface:ISession {c0447716-ff5a-4795-b57a-ecd5fffa18a4}
> ====================================================================================
>
> The first line says: "It was not possible to restore the virtual machine My
> VM". And
> "Resultaatcode" is simply "Result code", or something like that.
>
> It's (sadly) quite reproducible. Linux guess VMs included Linux Mint 19 and
> Debian 6.0.10.
> After that, I updated Linux Mint to 19.1, reinstalled the guess additions (I
> tried both
> the 5.x deb from Linux Mint 19.1 and the downloaded .iso), but with no
> change in behavior. BTW, downloading the .iso for Linux Mint 19 and 19.1 also
> fails at
> around 80% with an "Unkown error".
>
> I have run out of ideas on how to resolve the issue, and tried to rule out
> version conflicts.
> Of course, what I expect is to guest systems to just restore without this
> fail.
>
