LDAP authentication and enrolment use user "id Number" that is stored into mdl_user table at authentication time.
Some times the field mdl_user->idnumber (varchar(64)) length in not enough to hold idumbers of user belonging to a huge institution. Somewhere between moodle forums (http://moodle.org/mod/forum/discuss.php?d=56198) it is suggested to increase the lenght f that field from its usual size to 255 (ALTER TABLE mdl_user CHANGE COLUMN idnumber idnumber VARCHAR(255);)
Well, this is not enought. In spite of the bigger size Moodle continue to store ONLY 64 bytes. THIS LET THE LDAP AUTHENTICATION AND ENROLMENT USELESS for institution where the LDAP structure can't be simply changed for high level directives. 8-O