Unfortunately the change log of VirtualBox only contains one entry that may be related: “UDP checksum offloading in e1000 no longer produces zero checksums” (the VM uses a virtual e1000 network card).
Theoretically VirtualBox is open source but the source code is only available via SVN and the Web-UI isn’t really useful to list the changes and the SVN server itself is painfully slow 
Edit: I had to remove the links to to the changelog and the SVN as the forum does not allow it.