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		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.