Why Does SharePoint External Content Item Picker Not Select Item When OK is Pressed?

Question

I have a simple test scenario set up with 2 SQL Server tables.

TEST_MainTable
ID  Name    LookupValue
1   Blue    1
2   Red 2
3   Green   3
4   Yellow  2

TEST_LookupTable
ID  Description
1   Value A
2   Value B
3   Value C
4   Value D
5   Value E

I had to use Code Snippet to get the formatting of my table data to be readable above. Sorry, I'm a newbie at posting on this site.

I created a SharePoint External Content Type with all operations for both tables.

I created an association that maps the LoopkupValue column of TEST_MainTable to the ID of TEST_LookupTable.

I can create an external list for both content types and see and manipulate the data.

The problem is with the picker. When I use New Item or Edit Item, I can manually type an ID in the LookupValue field and hit the validation button and get the underline showing it validated my input. But if I open the picker and pick one of the lookup codes from the list and then hit the OK button in the picker, nothing happens. If I wait about a minute, the picker will go blank and the message "Connection has been reset" is displayed.

I have been trying everything I can think of for several days now to no avail. I have removed and recreated the ECT countless times. Recycled the app pool of the site. Tried it with out a foreign key on the SQL table. It makes no sense to me why the validation button will immediately underline the value (or give a red error message if the value is invalid) when pressed, but the picker will not load the item into the field when a selection is made and the OK is clicked. I am using SP Designer to do all the work. I have also gone into the model in Central Admin after creating the ECT and given Selectable in Clients permission to all Authenticated users on both ECTs.