Its doesn’t like empty disks., and fails if you include them: https://www.dmtf.org/sites/default/files/standards/documents/DSP0243_1.0.0.pdf Page 25 line 833:
The ovf:fileRef attribute denotes the virtual disk content by identifying an existing File element in the References element, the File element is identified by matching its ovf:id attribute value with the ovf:fileRef attribute value. Omitting the ovf:fileRef attribute shall indicate an empty disk. In this case, the disk shall be created and the entire disk content zeroed at installation time. The guest software
will typically format empty disks in some file system format.
If the ovf:fileRef is missing then an error points to a log file, the logs show a null reference. If ovf:fileRef is set to an empty string the installation succeeds, but without the empty disk, even though the installer indicates it is going to create one.