Dear Menocal,
looks like an entry into acs_rels for your tern (user_id, group_id, rel_type) already exist.
Please try the following query in postgres shell, replacing :user_id and :group_id with your actual integer ids:
select * from acs_rels
where object_id_one = :group_id
and object_id_two = :user_id
and rel_type = 'dotlrn_student_rel'
Hopefully you will obtain the entry that is preventing you from inserting the user again.
P.S. I could understand your question, but unfortunately my knowledge of spanish is not sufficient for a proper answer...
