virtualbox.org

Yeah, debian won't boot after I added it to a windows AD (windows server 2012)

I used this method: http://www.alandmoore.com/blog/2015/05/ ... directory/
Joining the AD was successful.

What could be the problem?

When I start the VM, GRUB runs and after choosing the system's version (restore or normal boot) a black screen appears.

I tried removing quiet mode from GRUB boot, no errors..

If your host will not boot then it has to be something that you added or changed. If this happened after you followed someone’s tutorial then you would need to ask them. VirtualBox will not cause this issue.

You might be able to boot in recovery mode and repair this but that too is outside the scope here.

It's the guest, not the host. I didn't do anything but joined the AD

You posted in Linux host, so is the host also Linux or something else?
Now you followed the tutorial and now the guest will not boot, correct? Then can I assume the guest worked before you followed the tutorial? If so this is not really a VirtualBox issue is it, and the same quotes would apply. Boot to recovery and repair or ask the person that you followed what went wrong.