Thanks mr IG! Sorry if I looked like an idiot...
But on the other hand, finally made a bit more progress; sorted out my " Don't know howto do 'set' " error.
(gdb) p ((citxmpp *)MyContext()->session_specific_data)->iq_client_resource
$2 = '\000' <repeats 255 times>
Client resource is certainly empty, that's why line 409 from serv_xmpp.c fails the condition and doesn't enter the "if" block!
And it's empty because... I didn't put a resource in the Pidgin client!
But I didn't because I thought Citadel's XMPP server supported a thing called "resource binding":
https://xmpp.org/rfcs/rfc3920.html#bind
Citadel is not able to "generate a resource identifier on behalf of a client", and currently can only send a strange (misleading for me) error saying "don't know how to do set".
So I just manually put a resource called "work" in Pidgin.
This gave me a different error I prefer to describe in a separate post.
