I was going to put this in the Bugs section... but it comes up with the same message as when you press Report User. Here is the message for both:

Rivière, you do not have permission to access this page. This could be due to one of several reasons:

Your user account may not have sufficient privileges to access this page. Are you trying to edit someone else's post, access administrative features or some other privileged system?
If you are trying to post, the administrator may have disabled your account, or it may be awaiting activation.

I only found out when I was coming to report a spam bot person the other day and that happened.

Thank you for reporting this, I have fixed this issue now so you should be able to use the 'Report a user' form again.