Having seen this issue a few times on my Notes Traveller 9.0.1 server that is used by BlackBerry 10 as ActiveSync source, thought I'd post error and fix.
Symptom: User is not receiving any emails on their BlackBerry 10 device although it is setup correctly.
Error in Traveler Server HTTP Log is: notes traveler BAD REQUEST (Syntax problem in the request or it could not be satisfied)
Fix: User had their Notes ID moved to a new OU but the Admin server in the ACL on the users mail file contains the wrong Admin server so the ACL is not updated. Notes still works as old username still in Name field but ActiveSync fails. Update ACL to reflect new username OU and fix the Admin server.