Problem with DataSet Relations

Hello ! I'm trying to configure a relation between two tables in a dataset. It works well when I open the form at the first time, but when I close the form and open it again (still in runtime), I get the following error message: "Faild to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints." I have no idea what table may cause this error and why it doesn't happen at the first run. I tried to dispose and clear the dataset when I close the form, but it didn't help. Thanks in advanced, Tom.