I thought it was just ME doing something wrong when I set up my forum, but I got this same error when trying to sign up on a friend's new forum.
I got this Invalid Signature error after I went to my email and clicked on the Verify Email Address. When I did the Resend it worked.
Thanks, will look into the possible cause of this.
I've signed up on a few more Forum Spark forums since I wrote the OP, and every time I've had to click Resend the email in order to get it. It happens immediately then.
Yep, I’m aware of it, had it happen a few times myself, but have not been able to look into it. Will move this more to the top of the to do list. 
Cedric said:Yep, I’m aware of it, had it happen a few times myself, but have not been able to look into it. Will move this more to the top of the to do list.
Or just change the text to say SEND EMAIL NOW instead of resend. Problem fixed for the time being :D
This has been fixed. When using the SSO to register, you will now automatically receive the email.
ForumSpark Support