This occurs also if you enter the "User Contributions" page through the "Special Pages"->"Wiki Data Reports"->"Contributions".
You enter the username of the person you want to see, hit "Go", and get the same error. No URL parameters appear in this case, presumably because this is now a POST, although the same error results (for easy to imagine reasons, from my perspective).
Is there some crossed wire in this page, mixing "user" with "userid"?
In any case, all contribs-by-user type functionality is "broken" as-is. The "workaround" is to manually edit the URL in the browser, which isn't all that great...
If you could provide some pointers, I'd dive into the code myself and see what's where, here? Heh.
Thanks.