After the account is created and a sucessfull conection is made to the server, you need to close down Opera and edit incomingN.txt (where N is the number for the account as seen in accounts.ini in the mail folder) in the mail folder. You want to set Force Single Connection to 1. See "Help -> about Opera Mail" for the location of the mail folder.
In the new mail account wizard, if you have an @yahoo.com, @ymail.com, @rocketmail.com, @yahoo.fr, @yahoo.no, @yahoo.co.uk, @yahoo.de or @yahoo.nl, all of that will be set up for you. If you don't have one of those email addresses for your yahoo account, put username @ one of those as your email address and then change it after the account is set up and attempts to connect (possibly retyping the username and password on the servers tab in the account's properties after you change the email address on the general tab).
I'm assuming that imap server address still works.
I remember Yahoo IMAP requiring some type of user agent header to be sent to the server (that mobile apps do). Opera did that too the last time I checked, so there should be no problem with that.
If yahoo has some time of 2-factor authentication and it's turned on, you might have to turn it off for Opera Mail to connect properly.
Ultimately, the log should tell you what's wrong.