Steps to reproduce:
- use a mobile phone with a browser on which you are not logged on on Commons, nor globally.
- go to Commons upload page through a Wiki Loves Monuments campaign link (I used one of the upload links from http://www.wikilovesmonuments.hu/participate.php )
- follow the registration link, fill out user name, password and captcha, and submit
I end up on a page with the URL https://commons.m.wikimedia.org/wiki/Special:CentralLogin/complete?token=9cadcab8a..., titled Central user log in; all its content is an error message in red saying "No active login attempt is in progress for your session". There is no indication that I am logged in (the mobile menu even has a "Log in" option), but the local login is actually successful, after navigating back, the upload works. I am not logged in on Wikipedia though.
I can reproduce with these exact steps, but if I use the campaign link with a desktop browser, or use a mobile browser bug don't use the campaign link, the error does not appear and the registration works.
(I don't see any relation, but best still mention that some moths ago I opted in to the mobile beta on some Wikipedias; never on Commons though.)
I wasn't sure where this bug belongs (CentralAuth? MobileFrontend? UploadWizard? WLM?) so I'm filing it here and hope someone can move it along.
Setting severity to major because this is exactly the workflow which most potential new contributors who arrive from the ongoing WLM campaign will go through. After seeing the error once you are logged in and the next upload attempt would work, but that's not what the error message suggests, so people might get discouraged from trying.