Error coming "Review all error messages above to correct your data. "

[ainteh09]

I have action plan template created with prepoulated "Assigned To" field. When I click on "Apply Action Plan" on account(person account) profile and then do lookup for template and select that it bring back all the values. After that I change some of assigned to values and some days after also. After that When I click on Save button then this error shows up. Review all error messages above to correct your data. " But there is no any message on the screen which shows any error message to understand why it is coming.

And weired bheaviour is that sometime it consider changes and save it and most of the time it stucks to this page by showing this error.

Is there any one who can help me this issue. We have UE of SF and using Action Plans - v3 - Unmanaged - EE, UE and DE.

thanks