It's probably because you've already read your quota of free articles for the week. Open it in 'incognito' mode and it should let you read it.
Thanks for that as a possibility, Dlk. It's not a site I visit that often and certainly not over the past week...
I could understand if it was just a register request to continue reading but this is a subscription request with a charge attached, which I never remember them having in the past.