The current work-around for the Digilent cable is shown below.
When creating/saving a project with a Digilent cable setup, go to the outputs toolbar menu in iMPACT.
Then select the "Cable Setup" option to bring up the "Cable Communication Setup" window.
Change settings if required, otherwise set to default and click OK.
Finally, save your iMPACT project.
This should give iMPACT enough details to create a working and uncorrupted .ipf project file.
You do not need the Digilent plugin installed for this to work.