it is a while i have this problem and how much i ask no one answers,
I have upgraded my maemo into the latest version, and installed pidgin 2.7.7 from its repository. when i try to login into my yahoo account in pidgin it says:
unable to validate certificate
The certificate for login.yahoo.com could not be validated. The certificate chain presented is invalid
and in conversation i've installed extra protocols. when i try to login into my yahoo account it don't do anything.
i'm using pidgin 2.7.9 on my pc without any problem.

plzz help me to fix it.

Not sure if this is related to your issue, but I'm also having a problem with signing into yahoo (using pidgin plugin for conversation and contacts 0.7).

When I try to sign in, nothing happens. Its just stuck doing nothing after clicking sign in. The weird part here is that it was working for a week or so then it just stopped.

Not sure if there are setting in the advanced section that needs to be corrected or so.

update: i was looking through the threads here and found a solution: removing scsa.msg.yahoo.com from advanced settings during sign in. I'm running latest stable cssu with pidgin protocal plugin for conversation and contact 0.7