Debian machine hangs after update virtualbox

[ZatarraNL]

Yesterday I updated my version of Virtualbox from 5.2.0 to 5.2.20. Since then one of my virtual machines has stopped working. Any backups I had don't work either. Downgrading to 5.2.0 doesn't work either...

The situation
Host: Windows 10 Pro
Guest: Debian 9
Version of Virtualbox: 5.2.20 (old version 5.2.0)
Version of Virtualbox Extension Pack: 5.2.20 (old version probably 5.2.0-118431
Version of Guest additions: none yet updated (old version VBoxGuestAdditions_5.2.1-118918)
Logs: see attachment
Number of virtualmachines: 2 (one boots, the other doesn't. Both are Debian 9)

What happens
The machine starts
GNU GRUB 2.02 loads
After choosing Debian GNU/Linux (with the quiet flag disabled) the following occurs:
- Loading Linux 4.9.0-8-amd64 ...
- Loading initial ramdisk ...
- 2 more unreadable lines (which disappear in an instant)
After which I get a black screen. It stays black for a very long time (hours).

Choosing any older debian-versions or any recovery mode doesn't work. I can access the GRUB commandline, for what it's worth.

What I've already figured out
I make regular backups of the .vdi file. Which in the past has helped me out. Using those files hasn't worked for me, so there must be something going wrong with the virtualbox update. Downgrading doesn't seem to do the trick either.
Maybe there's something wrong with the installed GuestAdditions... But I've no idea how to uninstall that part.
So far it's a lot of guessing...

I'm no linux expert, so please go easy on the reply and try to make it simple. This virtualmachine is very important for me (figuring the many internal and external backups), so help is very much appreciated.

[ZatarraNL]

I found the problem.

It seems that the update to 5.2.20 changed something in the settings of the virtual machine. Both machines were characterized as Ubuntu 32bit. Both where Debian, but one was 64bit and one was 32bit. So guess where it went wrong...

[socratis]

That's interesting (to say the least). I've been upgrading/downgrading between several VirtualBox version quite often (several times a week) and I've never seen the VM type change. Do you have a backup of an older edition of the .vbox files for the affected VM(s)?

[ZatarraNL]

It seems the VM's were not infected. It was only the setting of Virtualbox. After changing the settings in Virtualbox to 64bit the problem was solved: the machine started. The machine works perfect ever since.