Do you get any errors? Any output in the PHP logs? As a last ressort, you could edit .../moodle/enrol/database/enrol.php and remove the leading '//' from the error_log lines in the setup_enrolment() function to get additional error logging, to see what's going on.
Saludos. Iñaki.
Enrolment
Moodle 1.8 - Ext DB Enrolement problems
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