You probably have seen that Freenode has been getting lots of spam lately.
To protect against that some channels have activated a flag that only allows authenticated users to enter the channel.
If you're using the regular "nickserv" authentication way as I was doing, the authentication happens in parallel to entering the channels and you'll probably be rejected from joining some.
What you want is use SASL, that is a "new IRC" protocol that will first authenticate and then join the server/channels.
More info at https://userbase.kde.org/Konversation/Configuring_SASL_authentication.
Thanks Fuchs on #kde-devel for enlightening me :)
And of course, I'm going to Akademy ;)