SQL user problem"property IsLocked is not available for login '[username]'. This property may not exist for this object, or may not be retrievable due to insufficient access rights.(Microsoft.SqlServer.Smo)"

Hi,

I have SQL server (2005)

Some times without any changes users couldn't login to database 

When I open Management studio and click on specific user to see the user property it shows below error :

"property IsLocked is not available for login '[username]'. This property may not exist for this object, or may not be retrievable due to insufficient access rights.(Microsoft.SqlServer.Smo)"

All of the users shows that error.

Reseting SQL service is not sufficient and it requires server restarting 

After reseting server it doesn't have any problem

Would you please help me on this issue ?

Regards