Code:
http://mail.yahoo.com
https://mail.yahoo.com
https://mail.yahoo.com/d/folders/1
( https://login.yahoo.com/?.src=ym&.lang=en-US&.intl=us&.done=https%3A%2F%2Fmail.yahoo.com%2Fd%2Ffolders%2F1 )
no idea with regard to your methodology or environment,
Hermitkrab … however, when i log on to yahoo email:
email
password
uncheck "stay logged in".
there's talk among fellow netizens with regard to several factors … including one's a/v being the culprit … i won't point fingers anywhere.
for myself, happened just last week … unable to log into yahoo-email
(firefox). no screen-cap … seems to be similar to your issue. i tried a second browser
(chrome) with exact same sign-in parameters … same results. opened brave's browser … slowly typing in
"mail.yahoo.com" … before completing the address, the browser offered a slightly different url … which i chose.
logged on without a hitch. immediately afterward, carefully logged off and closed the browser. i then re-opened brave's browser … copying the url which allowed me to successfully log in. i then opened up firefox … pasted the copied url into address-bar … proceeding to log in as usual. no problem this time. and, ever since that time, i have gone back to the old url … without a hitch.
the link i've always used in the past did not have the
'https' security prefix … that could have been one problem
(doubtful). also couldn't be the browser. could be the a/v … however, i will never trust
'whitelisting' yahoo. honestly, i don't recall if there was a correlation between win-10 update or not. veritably, the issue could also be yahoo herself … updating servers … revising protocol … etc.
so,
Hermitkrab … try the three urls listed above. after each failed attempt … i strongly suggest for you to close/quit your browser … and then reopen browser for the next attempt.
p.s. the url in parenthesis … don't pay any mind to it … s'just browser syntax. 
