"unable to access record. This record is in use in Window "Adagio Inventory"
This message indicates a conflict with Inventory rather than Order Entry.

Have you completely turned off Opportunistic Locking?
Use \Softrak\System\chksys.exe to verify this.
I have seen exactly this error when opportunistic locking was on.

Before responding to you earlier I had checked in sample data by opening a detail on an order and a detail on a receipt to the same item at the same time. I also tested using two different user names. I can post the order when the receipt for that item is open, and I can post the receipt when the order for that item is open.