Before you export the OVA image from VirtualBox, eject the ISO file from the CD-ROM drive.
- In the Virtual Manager Console, go to Storage.
- Under Storage Tree, select the ISO file.
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Under the Attributes section, click the CD icon and then select the "Remove the disk from virtual drive" option.
- Export the virtual machine to the OVA image then import it again to DDI.
You should be able to import the custom Sandbox.