Not sure why, but my directory that connects to the Active Directory has failed me twice in 2 years. This is how my directory is setup:

1. AD binding user has Read Only permission to the AD.
2. The Connector settings:
   1. Use Nested Groups
   2. Use the User Membership Attribute
   3. Use memberOf for group membership
   4. Use Paged Results
   5. Use Naive DN Matching
   6. Enable Incremental Sync
3. Directory users User Object Filter to filter users.
   ( Everything else in the User Configuration section of the directory configuration is untouched. )

Out of a sudden, the directory can no longer synchronize to its cache. Support says it's a cache corruption due to changes to the AD.

Of course, the AD or other directories normally have lots of changes due to group usages, creation and removals. Also when people joins and leaves the company.

If the cache is incapable of keeping up with the chances, then i'd suggest to create a better fault tolerance cache or whatever that is causing the problem to cope well with changes to the AD or other directories.