1 – Open the files ‘.vbox’ and ‘.vbox-prev’ (if exist) files in any text editor and replace the first character of HardDisk uuid (take note to revert this change on step 6)

Example: nano /home/virtualbox/WindowsServer/WindowsServer.vbox

Change:

<HardDisks>
        <HardDisk uuid="{3ebaa9b6-8318-4b81-b853-8f30dd278bdc}" location="/home/virtualbox/WindowsServer/WindowsServer.vdi" format="VDI" type="Normal"/>

To:

<HardDisks>
        <HardDisk uuid="{2ebaa9b6-8318-4b81-b853-8f30dd278bdc}" location="/home/virtualbox/WindowsServer/WindowsServer.vdi" format="VDI" type="Normal"/>

2 – Reboot machine

Source: VirtualBox Cannot register the hard disk already exists – Stack Overflow