I have been plagued by the same VOD 603 error for months. It is not a HDCP2.2 or porting issue as Virgin suggest but a V6 problem which I have finally solved after trying everything possible over time.
Power down the V6 completely and restart, this usually brings back VOD, albeit temporarily. You may have to do this a few times to get it working. Then set the Power Option to "Always On" (there is little difference between power consumption in any setting).
Mine has now been OK since doing this pointing to the problem occurring when the V6 comes out of Stand By.
All on demand content produces this error on both of my Samsung TV s with one connect boxes. Reboot box and ok for about 24 hours.
Just for elimination purposes, make sure you have 1080 output enabled in your video settings. If there is an HDCP issue somewhere in your chain, this should at least enable the workaround and prove a point.
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I've previously tried setting everything to 1080P and still get the error. I've tried new cables. I've tried putting it through a HDMI splitter. None of this appears to make any difference. I will try changing my boxed to stay powered on permanently as the above suggestion would make sense as the problem always goes away following a reboot for however long I use the box and then would re-appear next day or after box has been in standby. I am concerned about the environmental impact of this and the cost of my electric bill as I have 4 boxes in total so would probably just have to put up with it and reboot every time I want to use VOD but it will be an interesting test.