PTURL%% error message when clicked

Hi,

I have a website which uses this plugin, but members need to register. However, when they click on the email it goes to the registration page, but with a red error message at the top that reads

‘It looks like you are trying to accept an invitation to join the site, but some information is missing. Please try again by clicking the link in the invitation email’

But this error appears even when I have clicked on the link in the invitation email. How can I fix this? I need to remove the error message or get it to say please register below or something like that?

Thanks