Thank you for your quick response.

How can I determine which value will cause this exception?

Below you can find the complete call stack.

BLToolkit.Data.DataException: validation error for column *** unknown ***, value "*** null ***"
At procedure 'FEEDINGS_UPDATE' line: 14, col: 3 ---> FirebirdSql.Data.FirebirdClient.FbException: validation error for column *** unknown ***, value "*** null ***"
At procedure 'FEEDINGS_UPDATE' line: 14, col: 3 ---> FirebirdSql.Data.Common.IscException: Exception of type 'FirebirdSql.Data.Common.IscException' was thrown.
at FirebirdSql.Data.Client.Native.FesDatabase.ParseStatusVector(IntPtr[] statusVector)
at FirebirdSql.Data.Client.Native.FesStatement.Execute()
at FirebirdSql.Data.FirebirdClient.FbCommand.ExecuteCommand(CommandBehavior behavior, Boolean returnsSet)
at FirebirdSql.Data.FirebirdClient.FbCommand.ExecuteNonQuery()
--- End of inner exception stack trace ---
at FirebirdSql.Data.FirebirdClient.FbCommand.ExecuteNonQuery()
at BLToolkit.Data.DbManager.ExecuteOperation[T](OperationType operationType, Func`1 operation) in C:\Projects\Computronic\SVN\Prowork\Components\bltoolkit.3.1\Source\Data\DbManager.cs:line 4009
--- End of inner exception stack trace ---
at BLToolkit.Data.DbManager.OnOperationException(OperationType op, DataException ex) in C:\Projects\Computronic\SVN\Prowork\Components\bltoolkit.3.1\Source\Data\DbManager.cs:line 521
at BLToolkit.Data.DbManager.HandleOperationException(OperationType op, Exception ex) in C:\Projects\Computronic\SVN\Prowork\Components\bltoolkit.3.1\Source\Data\DbManager.cs:line 4031
at BLToolkit.Data.DbManager.ExecuteOperation[T](OperationType operationType, Func`1 operation) in C:\Projects\Computronic\SVN\Prowork\Components\bltoolkit.3.1\Source\Data\DbManager.cs:line 4017
at BLToolkit.Data.DbManager.ExecuteNonQueryInternal() in C:\Projects\Computronic\SVN\Prowork\Components\bltoolkit.3.1\Source\Data\DbManager.cs:line 546
at BLToolkit.Data.DbManager.ExecuteNonQuery() in C:\Projects\Computronic\SVN\Prowork\Components\bltoolkit.3.1\Source\Data\DbManager.cs:line 2370
at Fds.Dal.DataAccess.BLToolkitExtension.FeedingAccessor.Update(Feeding obj)
at Fds.Dal.DataAccess.ManagerBase`2.Update(T obj) in D:\svn\FeedingSystem\trunk\Src\Fds.Dal\DataAccess\ManagerBaseT.cs:line 73
at Fds.Dal.DataAccess.ManagerBase`2.Save(T obj) in D:\svn\FeedingSystem\trunk\Src\Fds.Dal\DataAccess\ManagerBaseT.cs:line 92
at Fds.Presenter.ModelPresenter.SetFeedingDto(FeedingDto valueDto) in D:\svn\FeedingSystem\trunk\Src\Fds.Presenter\ModelPresenter.Feeding.cs:line 600