Bug is still present in Pidgin 2.12.0-1.fc25 (libpurple 2.12.0) (I don't
know why it is marked as closed).
To workaround it, you need to export intermediate authority with you
browser or openssl.
For example with firefox: browse to the site, [https://support.mozilla.org /en-US/kb/secure-website-certificate#w_problematic-certificates view
certificat], got to Details tab, choose the second to last certificat, and
Then copy the autorithy certificat to /etc/ssl/certs/
And restart pidgin: it may ask you SSL server issued by the authority
once, but won't ask you each time it reconnect like before.