When I sent my Vertical Power VP-50 back after my failed attempts at upgrades, I included the cable you have to make to accomplish upgrades in with the package, with the hope, actually, that it would reveal a problem. Nothing else explained the stalled upgrades, and yet when I checked continuity, everything checked out fine. I didn't want the problem to be either the control unit (CU) or switch unit (SW), nor any incompatibility with the computers I'm using to update.
Fortunately, John at Vertical Power found the problem -- it was, in fact, the cable I made. He upgraded the unit, and is sending it back, with a new cable, too. You can't beat that service with a stick!
As it turned out, it's just as well I had to pull the system out and send it down to Albuquerque for some professional help. I made some changes in the system's location, moving it closer to the center of the subpanel assembly to move it away from a wiring run. And I also changed the way it's mounted, adding some nutplates so that it now screws into the brackets that hold it in place, rather than bolt it in place. And I added a nutplate or two to put some Adel clamps on the brackets to assist in some wiring runs.
In the meantime, I determined that the Grand Rapids EIS 4000 isn't going to provide the visible fuel level monitoring that I want (although it will provide flow rate information, which is possibly more valuable), so I ordered an Electronics International dual fuel level indicator (about $400) for the panel, instead. And having the VP-50 system out will make it easier to drill the holes in the panel to insert it.
Plus it has pretty colored lights.