Sorry to hear this Simon!
Licenses work fine for me on my local test site, so I’m curious – can you share your “System Information” with us? I’ll take a look at that and see what I can learn (if anything) to try and reproduce the error.
Also, if you go http://theeventscalendar.com/my-account/license-keys, does the URL on which you tried activating the license show up there? If so, try using the “Disconnect now” link next to the URL to disconnect the license; then see if adding the license on your site works any more successfully.
Thanks,
George