Microsoft doesn't know why (or have a fix), but Cluster Server does not support more than 900 shares.
When you create more than 900 shares on a Cluster Server, you may started getting various RPC errors. You will get one or more of the following System Event Log entries:
1016 (source CLUSSVC)
1065 (source CLUCSSVC)
1000 (source CLUSSVC)
1069 (source CLUSSVC)