Hi Victor,
Thanks so much for getting back to me.
I’ve shared the system information with you.
Attached are a couple of screen shots of the event in edit mode. Let me know if you need more/different shots.
Since I posted, I’ve tried rolling back to an earlier back-up but it made no difference.
I’ve also realised that I can’t reproduce the error consistently. Sometimes it works fine. Sometimes it doesn’t. And while I’ve been working on this a couple of people have been able to register, while others haven’t.