He trobat a un altre forum aquesta resposta, quan algú tenia un problema similar:
AFAIK "sesskey" is a randomly generated number to ensure that calls to urls in your moodle system are from people who are logged in to the system and not from hacckers. From the moment one is logged in and for the duration of that session (not sure of the definition of session, but 'log in period' might approach the true the meaning of the term) the user is passed randomnly generated numbers with each click, that must be passed back to the system each time one does anything interactive. Moodle keeps a record of which random numbers it has passed out to those logged in and rejects system calls from those that have an invalid session key.
Sesskey was introduced from about moodle 1.4.4.
The "required parament (sesskey) was missing) usually arises, in my experience, when you are trying to use an older module, or an older download of a module, with a new version of Moodle.
... La qual cosa no lliga molt, perquè el curs va ser posat a disposició a finals d'agost!
Continuo buscant...
Andreu.