Data Source Conflict

If you are developing BCS solutions in Visual Studio, you might have experienced the following error when you attempt to create or update a new item in an external list:

DataFormWebPartException: Data Source Conflict: Your changes conflict with those made concurrently by another user. Refresh the form and submit your changes again.

Solution: In your Visual Studio 2010 BCS project, find the Create or Update method that's failing. Verify that each of the fields in your input type descriptor have the Creator property (for the Create method) or Updater property (for the Update method) set to True.
Note, this property should not be set to true for the identifier field if the LOB auto generates the value. You also need to delete and re-add the external list before the changes take effect. (Because the object definition of the external content type is saved in the existing list and does not change when you update the BCS model.)

BCS - add item form for creator method not working

You have created a BCS model, added a creator method to it but it is throwing an Runtime Error. In the logfile you find "SPException: Unable to find the default new form for list"

A probable reason for that error is the following. You created the model only with a ReadList and ReadItem method. Then you created a list in SharePoint and tested that functionality. Then you added a creator method and used the previous created list to add an item. That does not work because the list was provisioned without the "new form". Just delete the list and create a new one. The "new form" for the list should show up as expected.

Access denied. You do not have permission to access this content

You have created an External Content Type in SharePoint Designer 2010 with the authentication mode "User's Identity". When you create a list and want to access the data in your portal, you get the error message "Access denied. You do not have permission to access this content" or "Access denied by Business Data Connectivity".

You need to set permissions in Central Administration under "Manage Service Applications" in the "Business Data Connectivity Service Application". Click the BCS service and configure the permissions in the context menu of your ETC.

Also the user needs permissions in your external database.