Bugzilla

I finally noticed above that you are using the app specific password that yahoo generates. I was using the oauth2 that seems to now be supported. I changed my yahoo on tb account back to app specific which just uses TLS and the 16 character password that yahoo generates. I also told tb not to save the password, which seems to be what you are doing too. After selecting folders the prompt appears and retry seems to continuously fail. 

You didn't mention this but after a couple retries i see a pop-up notification that the connection has been lost. Looking with wireshark, I see that yahoo does drop (RST) the connections. 

I don't see that tb is doing anything wrong however. Yahoo seems to quickly drop connections when there is no activity (after about 5m) where most servers will go about 30m before disconnecting. Then when you click on a folder and tb tries to restore the connection, yahoo rejects the login and after a few retires, yahoo drops the connection. When this occurs there are only about 3 active connection.

One way to fix this is to wait a few minutes. This causes yahoo to logout (imap wise) several connection and free resources so new connections and imap logins can occur again. Looking with wireshark, during this few minutes, several connections are closed by yahoo so new ones can occur again.  Other non-yahoo imap servers don't seem to have this issue with timing out, dropping connections and allowing new connections and rejecting imap logins.

Another workaround that seems to work for me is to reduce the number of cached connections down to 3 (Server Setting/Advanced). I saw the  problems with 4 and 5 but, so far, not with 3. Let me know if this helps for you.

I finally noticed above that you are using the app specific password that yahoo generates. I was using the oauth2 that seems to now be supported. I changed my yahoo on tb account back to app specific which just uses TLS and the 16 character password that yahoo generates. I also told tb not to save the password, which seems to be what you are doing too. After selecting folders the prompt appears and retry seems to continuously fail. 

You ~~~didn't~~~ mention**ed** this ~~~but~~~ after a couple retries i see a pop-up notification that the connection has been lost. Looking with wireshark, I see that yahoo does drop (RST) the connections. 

I don't see that tb is doing anything wrong however. Yahoo seems to quickly drop connections when there is no activity (after about 5m) where most servers will go about 30m before disconnecting. Then when you click on a folder and tb tries to restore the connection, yahoo rejects the login and after a few retires, yahoo drops the connection. When this occurs there are only about 3 active connection.

One way to fix this is to wait a few minutes. This causes yahoo to logout (imap wise) several connection and free resources so new connections and imap logins can occur again. Looking with wireshark, during this few minutes, several connections are closed by yahoo so new ones can occur again.  Other non-yahoo imap servers don't seem to have this issue with timing out, dropping connections and allowing new connections and rejecting imap logins.

Another workaround that seems to work for me is to reduce the number of cached connections down to 3 (Server Setting/Advanced). I saw the  problems with 4 and 5 but, so far, not with 3. Let me know if this helps for you.