I just recently had the same issue and a colleague suggested I check the field lengths in the import set table. What worked was increasing the field length for "manager" to 250 from the default which was 40.
Under Tables look at the User(imp_user) table and see what the field length is. We also bumped up source, dn and bumped memberOf way up (2000) as that value can be quite long.
This may not work for you since you've been using SN for a while but might be worth checking.
Regards,
John
