A 401 error doesn't have anything to do with the PPRuNe forum. I suspect that you are linking to a page that is visible to you because you are logged in on that site, but when the PPRuNe software tries to access it, the error is produced.
Pretty technical explanation here:
https://airbrake.io/blog/http-errors...thorized-error but there's nothing you can do to sort this, other than not linking to that particular page. Try posting a link below for testing purposes.