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Hi, I have the same issue. Don't use my yahoo account much, but when I checked it my problem is the same. I tried to connect on my android phone as well and it said it couldn't connect, although a recent email appeared in the inbox. But I can't send from it. Two step verification is disabled and use of less secure apps is permitted.
Here we were discussing the Operamail desktop, in your Android phone you have to use the official and updated Yahoo app.
I switch to auth plain in the dialog and try again. Then it connects fine.
Thanks, now it's working for me too using "Auth plain".
When the problem started I had tryed all the autenticantion modes without any solution. Probably the Yahoo team have solved the issue.
Thanks to all for support. 
You can try creating a log file to see if it shows something that may help you find the cause of the issue.
This is the log:
==== Logging started ====
07/08-2017 22:19:58 IMAP#1/1: Connecting...
07/08-2017 22:20:01 IMAP#1/1 IN {}: * OK [CAPABILITY IMAP4rev1 ID MOVE NAMESPACE X-ID-ACLID UIDPLUS LITERAL+ XYMHIGHESTMODSEQ AUTH=plain SASL-IR AUTH=PLAIN AUTH=LOGIN AUTH=XYMLOGIN AUTH=XYMPKI AUTH=XYMCOOKIEB64 AUTH=XOAUTH2 AUTH=OAUTHBEARER] IMAP4rev1 Hello
07/08-2017 22:20:01 IMAP#1/1 OUT {}: 0000 ID ("name" "Opera Mail" "version" "1.0" "vendor" "Opera Software ASA" "os" "Win32")
07/08-2017 22:20:01 IMAP#1/1 IN {}: * ID ("remote-host" "2.40.184.37" "vendor" "Yahoo! Inc." "support-url" "http://help.yahoo.com/" "name" "Y!IMAP" "host" "sky700102.imap.mail.yahoo.com" "version" "1.1.10284")
07/08-2017 22:20:01 IMAP#1/1 IN {}: 0000 OK ID completed
07/08-2017 22:20:01 IMAP#1/1 OUT {}: [command contains password]
07/08-2017 22:20:01 IMAP#1/1 IN {}: +
07/08-2017 22:20:01 IMAP#1/1 OUT {}: [command contains password]
07/08-2017 22:20:01 IMAP#1/1 IN {}: + eyJzdGF0dXMiOiJpbnZhbGlkX2NyZWRlbnRpYWxzIiwic2NoZW1lIjoiYmFzaWMiLCJzY29wZSI6Imh0dHBzOi8vbWFpbC55YWhvby5jb20vIn0=
07/08-2017 22:20:01 IMAP#1/1 OUT {}: [command contains password]
07/08-2017 22:20:01 IMAP#1/1 IN {}: 0001 BAD [AUTHENTICATIONFAILED] AUTHENTICATE Invalid credentials
07/08-2017 22:20:01 IMAP#1/1: Disconnected: Received disconnect order
07/08-2017 22:20:05 IMAP#1/1: Connecting...
07/08-2017 22:20:05 IMAP#1/1 IN {}: * OK [CAPABILITY IMAP4rev1 ID MOVE NAMESPACE X-ID-ACLID UIDPLUS LITERAL+ XYMHIGHESTMODSEQ AUTH=plain SASL-IR AUTH=PLAIN AUTH=LOGIN AUTH=XYMLOGIN AUTH=XYMPKI AUTH=XYMCOOKIEB64 AUTH=XOAUTH2 AUTH=OAUTHBEARER] IMAP4rev1 Hello
07/08-2017 22:20:05 IMAP#1/1 OUT {}: 0002 ID ("name" "Opera Mail" "version" "1.0" "vendor" "Opera Software ASA" "os" "Win32")
07/08-2017 22:20:05 IMAP#1/1 IN {}: * ID ("remote-host" "2.40.184.37" "vendor" "Yahoo! Inc." "support-url" "http://help.yahoo.com/" "name" "Y!IMAP" "host" "sky700125.imap.mail.yahoo.com" "version" "1.1.10284")
07/08-2017 22:20:05 IMAP#1/1 IN {}: 0002 OK ID completed
07/08-2017 22:20:05 IMAP#1/1 OUT {}: [command contains password]
07/08-2017 22:20:05 IMAP#1/1 IN {}: +
07/08-2017 22:20:05 IMAP#1/1 OUT {}: [command contains password]
07/08-2017 22:20:05 IMAP#1/1 IN {}: + eyJzdGF0dXMiOiJpbnZhbGlkX2NyZWRlbnRpYWxzIiwic2NoZW1lIjoiYmFzaWMiLCJzY29wZSI6Imh0dHBzOi8vbWFpbC55YWhvby5jb20vIn0=
07/08-2017 22:20:05 IMAP#1/1 OUT {}: [command contains password]
07/08-2017 22:20:06 IMAP#1/1 IN {}: 0003 BAD [AUTHENTICATIONFAILED] AUTHENTICATE Invalid credentials
07/08-2017 22:20:06 IMAP#1/1: Disconnected: Received disconnect order
I tried all suggestions (by myself before asking here) but nothing is working.
I have done the log but if I post it the system tells me a moderator have to approve it.
I have retyped and checked all without any solution.
But if you don't use it how you can help?!?!?!
I don't know if Yahoo has changed something (they refuse Opera??? In this case Opera team have to investigate with Yahoo team).
I have retyped user and password, i have also created a new account to try... Nothing has changed.
I have not enabled 2nd factor or similar, I never access my account in the browser, I use only the client for the mail.
I noticed that trying to download mails Operamail gives me an error for not known certificate by my AVG antivirus. If i accept it, the program go on and gives me the errors i wrote, if i don't accept it the program ask me again. I have also disabled AVG and so no certificate are asked to be accepted but the errors is always the same.
But you have a yahoo account that you read with Operamail without problems now?
Done, nothing changed.
AUTH CRAM-MD5: Aurhenticate mechanism not supported.
AUTH LOGIN: (the usual method) AUTHENTICATE Invalid credentials.
AUTH PLAIN: AUTHENTICATE Invalid credentials.
And the other two options: LOGIN Invalid credentials.
I have some e-mail address with Yahoo (personal, work...) and until the last week I used them without any problem with Operamail (and Opera 12.17) but in the last days (about 1-2 weeks) doesn't connect to Yahoo IMAP and I receive an error "Authenticate Invalid credentials".
Obviously the credentials are the same, nothing has changed, and I can access my accounts using the browser with the same credentials.
Waiting for a solution as soon as possible.
Thanks, Igor
Beh, l'idea di Vivaldi è proprio quella.
Intanto il 15 febbraio 2016 è uscito Opera 12.18 (solo per Windows), peccato non ci siano in giro informazioni circa questo aggiornamento e quali correzioni di bug porta. Qualcuno ha informazioni in merito?
http://ftp.opera.com/ftp/pub/opera/win/1218/