After reading T169261 I've tried to reset my password (it was indeed very old) and my issue has been resolved. However, users without provided email address won't be able to use this feature.

Other case.

There is a report at it.wp as well, but no additional details from user yet.

For anyone affected by this problem who wants to help track it down, I'm going to code a list of things to try (and potentially report back):

In T169261#3452695, Anomie wrote:

Authentication bugs are notorious for having similar symptoms with different causes, since the symptom is usually "I can't log in" or "I get logged out unexpectedly" and there are many things that can cause that, including actual MediaWiki bugs like this one, browser bugs like T151770, mysteriously-corrupted cookies that never get enough information to be investigated, and user error such as blocking first- and/or third-party cookies from our sites. And then people tend to assume their problem is the same as someone else's.

Which is why we usually wind up asking people with authentication problems to try several things:

• Report exactly what happens, since "login submits successfully but I'm not logged in", "I'm logged in for the first page-view but logged out when I click any link", "'a loss of session data' when trying to log in", and so on all point to different causes. The last can even point to different things depending on when exactly in the process it occurs.
• Clear all cookes and try again, to see if it's screwed-up cookies.
• Log in using a different browser/device, to see if it's something screwed up in the browser.
• Log in to a different account, perhaps even a newly-created account, to see if it's something wrong with the original account.
• Capture the HTTP requests and responses so we can look for subtle signs of how exactly things are failing.

[...]

• Assuming "Manuela bertola" is the user name in T168858#3451751, that does look like an instance of T169261 too.

No more reports since 19 July... is this still UBN? Per Anomie's comment quoted above, it's possible that this may have been fixed by virtue of fixing T169261.