UPD The problem is presumably fixed at the time of 12:30 Moscow time.

Major failure in the services of the Russian Internet giant. On the morning of September 11, 2018, Internet users reported about the inaccessibility of the main web version of Yandex.Mail and Mail for the domain. The problem occurs when trying to authorize through the Yandex Passport regardless of the browser version.
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Yandex technical support recommended using a
light version of the mail service.
The company did not comment on the reason for the failure, but confirmed that there are "some difficulties in entering the mail":
On the
DownDetector website,
you can see the crowded sites of users who experience difficulties with Yandex services.

The previous major failure at Yandex occurred
on August 23, 2018 , but its scale was more modest, although it lasted much longer.