Hi,
I have checked the error logs and there is no error of such kind showing.
I've also got in touch with support who say
"Thanks for getting in touch. If an error is outputted by your script, all errors will show in the same error log section.
If your site doesn't output a 500 error, it won't show here."
So basically it does go to Twitter and then I get the error, maybe the callback is still wrong. I'm using https and my forum is at domain.com/forum for example.
So I have set the call back to be https://mydomain/forum/auth/twitter so I can't see why this isn't working.
Many thanks
Jon