SharePoint 2010 Claims Authentication Unexpected Error: The security token username and password could not be validated

I have configured Claims and FBA in SharePoint 2010, with a custom provider for a SQL Database. Everything works fine. Users can login.

But if I enter an existing username with a wrong password and I try to authenticate, an exception is registered in the log:

Password check on 'user_id' generated exception: 'System.ServiceModel.FaultException`1[Microsoft.IdentityModel.Tokens.FailedAuthenticationException]: The security token username and password could not be validated. (Fault Detail is equal to Microsoft.IdentityModel.Tokens.FailedAuthenticationException: The security token username and password could not be validated.).'.

The behaviour is correct, the user can not login, but it seems strange to me that an unexpected exception is registered. This error is registered by SharePoint, our custom membership provider works fine and does not register anything.

Is there anything we have not done well?

Thanks,