In page Add_Product_UI, you must have one and only one dataview with data source Context and entity Product.
I had the same problem. For me it helped to remove additional parameters, that I had accidentially created before, in the ‘Edit page parameters’ window.
Thanks for this tip on removing additional and unneeded page parameters!
That removed finally the CE1620 error.
I spent quite a long time trying to find the "Edit page parameters", which is exactly at the top of the page. The "Parameters" icon is the yellow demi-square, demi-triangle (off page connector flowchart symbol), exactly as illustrated in the above screenshots.
Many thanks again!