I'm having the same problem. I've got a new PC recently and tried watching BT Sport last month. It said I'd used up all devices and would need to wait till the beginning of this month. But I'd definitely not registered 5 devices in total. So I can't see how cookies were the problem as I'd never registered this PC as one of my 5 devices.
I tried again to register the device/PC tonight so I could watch the football and it indicated I could make 3 changes this month. I tried Registering again and it tells me I've reached the maximum number and need to wait till start of next month. Not sure how my allowed 3 changes have been used up.
I don't really want to wait another 4 weeks to make my next attempt but doesn't seem like there any other option.
The number of complaints indicate it's not a user issue but a problem Virgin needs to sort out.