error : 18452 in clustering; very urgent

We have 2 failover servers and one client in our test environment. We have all the three configured in mixed mode authentication(windows + sql server authentication). But when we are failing over from 1st server to 2nd one, we are getting the following error message,
'Unable to connected to server xxxx: Server : Msg 18452, Level 16, State 1 [Microsoft][ODBC SQL Server Driver][SQL Server]Login failed for user 'null'. Reason: Not associated with a trusted SQL server connection.'

I got some clue.

For instance we have ser1 and ser2 as clusters. I registered both in the client's EM. Now what I did was, I made ser1 to run and stopped sql service in ser2 (active/passive) through failover. Now in the client's EM I'm able to open ser1 and I cannot access ser2. Now what I did was, I failed over to ser2 ( so sql service in ser1 is stopped). But now I can still be able to access ser1 and also I can access ser2, even I'm able to open query analyzer in ser1 through EM.But when I try to open query analyzer directly, it is giving the same error message. ser1 is the virtual name given to both ser1 and ser2. So I think when connecting through EM, failover is occuring fine.I'm just wondering why is this happening only with dsn. Can anybody help me?

You can get this particular error in a number of scenarios -- the common scenario is when one attempts SQL authentication using an NT login account. If the system is unable to find the login account it will raise this error. It is not clear the scenario behind this error.

|||The information you provided is not sufficient to diagnose the problem. Do you get this error only after failover? If you start the SQL Server serive on node ser2 (using net start and not via cluster admin) do you still get the same error when the client is also running on node2? Do you get this error only when the client is running on a different node than the SQL Server?
The error will be raised when SQL authentication is tried using an NT login account and it seems like the system is not able to find the login account. Make sure you are using the same account to connect after failover.