It seems there was a change in the MySQL settings when my site was migrated to the new server last weekend. This change was causing orders made using the Paypal Express Checkout button to display an error instead of the confirmation page. The orders and payments were processed correctly, hence my not noticing the issue.
I was alerted to this issue yesterday evening (thanks Peter) and have now had my hosts change the MySQL settings to fix it. I’ve done a few tests and everything is back to normal and the error page should not show.
Please let me know if there’s eve any unusual behaviour on the site – the sooner I know about it the sooner I can get it fixed.