VirtualBox - Snapshot didn't start - ahci#0: The target VM is missing a device on port 0

I have set up a virtual machine with VirtualBox like a year ago.The OS is Ubuntu 16.04. Today, I've tried to start the snapshot like always. VirtualBox loads the saved state till 100% but then I've got the following error message:

    Für die virtuelle Maschine Ubuntu 16.04 konnte keine neue Sitzung eröffnet werden.

ahci#0: The target VM is missing a device on port 0. Please make sure the source and target VMs have compatible storage configurations [ver=9 pass=final] (VERR_SSM_LOAD_CONFIG_MISMATCH).

Fehlercode:E_FAIL (0x80004005)
Komponente:ConsoleWrap
Interface:IConsole {872da645-4a9b-1727-bee2-5585105b9eed}

Obviously, the VM lost it's connected *.vdi, when I open, the menu to configure the vm no disk is mounted (the option to add a disk is greyed out). The only thing actually mounted is the VBoxGuestAdditions.iso

Does anyone know, how I can get the vm running again?

VirtualBox Version: Version 6.0.8 r130520 (Qt5.6.2)

Host OS: Windows 10

Guest OS: Ubuntu 16.04

If you need more information, just let me know.

The core of the problem for me was that virtual storage somehow got detached, so I had to re-attach it manually. Here's what I did:

• Go to the Configuration (Ctrl+S) of virtual machine
• Select Storage
• Click on icon Add Hard Disk (IDE or SATA)
• Select Choose an existing disk
• Select the file *.vdi created for your virtual machine, in the right folder
• Valid with OK
• Start your virtual machine

I also posted my problem and solution here.