Sql server failover clustering can not failover to another node suddenly

I have set up two ACTIVE-ACTIVE sql server failover clusters using two physical win2008R2 servers (four sql instances, both servers have an active instance, and also another standby instance).

It's online for at least two years.

It all went well until suddenly someday one cluster failover to another server, that means two sql server instances are running on one box.
when i try to failover back manually, it always ended up failed. i try many times, no luck.

The windows event log during my manual failover operation which tell the operation failed.

- <Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">

- <System>
  <Provider Name="Microsoft-Windows-FailoverClustering" Guid="{BAF908EA-3421-4CA9-9B84-6689B8C6F85F}" /> 
  <EventID>1069</EventID> 
  <Version>0</Version> 
  <Level>2</Level> 
  <Task>3</Task> 
  <Opcode>0</Opcode> 
  <Keywords>0x8000000000000000</Keywords> 
  <TimeCreated SystemTime="2013-08-05T15:29:38.026463100Z" /> 
  <EventRecordID>124624</EventRecordID> 
  <Correlation /> 
  <Execution ProcessID="3004" ThreadID="9112" /> 
  <Channel>System</Channel> 
  <Computer>DB-CLUSTER1-1.mycompany.com</Computer> 
  <Security UserID="S-1-5-18" /> 
  </System>
- <EventData>
  <Data Name="ResourceName">SQL Server (SQLCLUSTER02)</Data> 
  <Data Name="ResourceGroup">SQL Server (SQLCLUSTER02)</Data> 
  </EventData>
  </Event>

i dont know how to troubleshooting this failover issue. Any help would be appreciated~

updated:

here 's the event log when all this started:

Could not register Service Control Handler. Operating system error = 2310 (Shared resource not exists)

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