VW T5 gearbox problem

This problem has made the vehicle unmovable , Its' a real mind bender. As per the title;

The engine is good; try to engage reverse and the engine stops instantly, to drive forward you need to pass through reverse selection, so going through reverse is necessary, if only momentarily.

It is presently with a VW main workshop.  Been there for three days under investigation, no solution !
Has anyone any experience of this problem ?

I THEORISE (not welcomed by VW workshop !)

It's a CANbus wiring system, not (vaguely) unlike the ringmain for 13 Amp domestic wiring
I postulate that there is a serious problem in the canbus to gearbox link which crashes the whole canbus system including engine control.

Has anyone had experience of this type of problem.
This is an appeal to the cognocenti or any poor chicken who has paid a garage bill recently.

Keith Roberts 
currently confined to barracks.

Manual or auto? If it's an auto then the codes stored in the gearbox controller may give you hints. You should get all of the gears lit or flashing on the dash display if fault codes are stored. But that won't apply with a manual box, which has no controller (but now I read more carefully, it must be an auto, I think).

The theory about the CAN network might have some validity BUT the CAN network is very robust - it's a 2-wire network, and a single broken wire won't stop it from working. And if you have had a single broken wire for a while and now the second wire has broken, you should have been getting fault codes for ages beforehand.

If the drivetrain CAN network fails, you will see plenty of evidence of it. You don't say what year, but assuming it's pre-facelift, the ECU, the ABS controller and the gearbox controller will all log fault codes if the drivetrain CAN network goes AWOL. This will be immediately evident to the VW techs, unless they are complete idiots.

Did you check to see if the reverse lights come on while it's in reverse gear?

Thanks for your response Roopert.  VW are still scratching their collective head.  They have been pondering for four working days. I will post a report if/when they find the evidence they are looking for.
Keith

One problem with main dealers (VW or any other) is that their "technicians" are completely dependent on the fault being one that the specialists at VW in Germany have thought about. If (for example) it were a failed chassis electrical connection which meant that power to the reverse lights brings the whole of a control module's ground connection up to 12V (thus in effect disconnecting it from 12V power) they would probably consider that so unlikely as to never document it.

But, having just re-read, I think your van must be a facelift with a DSG auto. If so, I would want to see the diagnostic codes from the 'box - it's very complex, and it would not surprise me to hear that the box can command an engine stop under certain fault conditions. Again, you would see an odd gear selector display on the dash if that's what is happening.