Could not verify the content of 'Windows 7 x64.mf' against the available files (VERR_MANIFEST_FILE_MISMATCH). Result Code: VBOX_E_FILE_ERROR (0x80BB0004) Component: ApplianceWrap Interface: IAppliance {8398f026-4add-4474-5bc3-2f9f2140b23e}

Hi, i had the same problem with the version 3.1.4. According to a comment found here https://forums.virtualbox.org/viewtopic.php?f=5&t=26617
the system check the manifest file just if it exist. What i did was simply remove the .mf file from the directory where the .ovf and .vmdk files are and the import works fine.
This is not a solution but at least you can import your stuff.