If I open
https://tools.wmflabs.org/xtools-articleinfo/
and then click on "Log In" which points to
https://tools.wmflabs.org/xtools/oauthredirector.php?action=login&callto=https://www.mediawiki.org/w/api.php&returnto=https://tools.wmflabs.org/xtools-articleinfo/
I'm redirected to something like
https://www.mediawiki.org/w/index.php?title=Special:OAuth/authorize&oauth_token=....&oauth_consumer_key=...
where I can click "Allow". At this point I get
Failed to initiate Authorization!
Returned error:
Error retrieving token: mwoauth-oauth-exception