Thanks Mike. From my conversation with TfL it seems the glitch lies with their payment provider. At least I’ve encountered nothing similar before – I used an older version of this card last year to buy my current travelcard, so perhaps there was some tweak in the SCA since then.
I’ll try to renew at a Visitor Centre on Wednesday and report back – thank goodness they still exist!
And thanks for the advice on TOC smartcards – I’ve also found the FAQ page on the Key which mentions in passing that travelcards on it must be issued from outside the zones, which must exclude a large number of Southeastern passengers. I guess it protects the integrity of Oyster but this mess of rival smartcards just perpetuates needless complexities…