Hi Alan,
That’s an interesting sequence. I do know that even paper travelcards object to being used in the same gates in quick succession. It’s called passback and the system tries to prevent you passing the ticket back for another person to use.
I think your smartcard may well have worked on the gates, rather than the POM at Rickmansworth. It would probably be worthwhile asking GA if they know what had happened. It might also be worth asking TfL for a refund of your Oyster charges given that you had a valid travelcard.
I’m wondering whether the block was just for the Heathrow station that you’d been ‘abusing’ and was cleared as soon as you used it on a gate somewhere else. It’s certainly not very friendly to stop a perfectly valid ticket from working.
Another possibility is that the Underground POM didn’t understand something the gates had done to the card, given that the ITSO readers said that the ticket was fine.
Let me know what GA and/or TfL say.