OK I've discovered one thing: the table ofGojaraSessions wasn't created because the SQL is wrong (at least for MySQL):
ERROR 1170 (42000) at line 2: BLOB/TEXT column 'username' used in key specification without a key length
So I've modified the gojara/database/upgrade/1/gojara_mysql.sql to have a limit on the indices (PRIMARY KEY (username(255), transport(255))).
The original problem still exists, though. I can't get the transports to work as stated in my original posting.
One thing I've also noticed is that when performing a clean registration to one of my Spectrum2/Gojara transports, the Spectrum2 log shows:
2013-07-30 21:53:55,502 INFO AdminInterface: Message from admin received: 'unregister myuser@mydomain.tld'
2013-07-30 21:54:01,962 INFO AdminInterface: Message from admin received: 'unregister myuser@mydomain.tld'
2013-07-30 21:54:02,102 WARN RosterResponder: myuser@mydomain.tld: User is not logged in
Whereas myuser is my real jabber user and mydomain.tld my FQDN.
Update:
I've managed to get it working partly, by removing gojaraadmin@mydomain.tld from admin_jid and adding my own jabber ID. Somehow the gojaraadmin user removes my registration immediately.