Multivalue Combobox on m:n CDS relationship gives "Value must be a data entity record" on submit
I am trying to implement a simple Object-Subitems m:n relationship in a canvas app. So I start from Data with Table "Object", add the table "Subitems" and go to the Edit Screen.. so I add the Subitems-Field and get Error on Choices(_), which I change to Choices([@Object].Subitems) and all works even default Values are shown in the multi value combobox, until I submit the form, it gives Error "Value must be a data entity record". I also tried the lookup field Choices([@Object].cr9b0_object_cr9b0_subitemswhatever) field, with no other effect.
How do I correctly relate the subitems to the object with a multi value combobox in a canvas app?
Please help, I couldn't find anything working yet and I have been searching for days now.