Badly written mag stripe info at the issuing ticket machine. I've had similar issues with a travelcard issued on a portable machine by an
SWT▸ 'queuebusting'
RPI▸ at Fareham, it wouldn't operate any barriers at all in London.
Paul
Possibly bought on by a software change in either or both the issuing machine, (not writing the strip correctly) or the Salisbury gates (not reading what Warminister has written).
I've had this with a Gold Card from Taplow which stopped opening
LUL▸ gates. Turned out LUL had altered the software and Taplow hadn't been given the amendment to change it's writing.
A very good chief programmer I worked with always asked the question what's changed, when something went wrong? Invariably it was the change that had caused the problem