E1: R42800 fails when AAI RC with blank BU and PO#5=3

Hello,

I have settled the AAI RC with a blank BU and I have populated the processing option #5 G/L Account Business Unit [Tab. Defaults] of R42800 with the value 3 (use the Revenue Business unit for all accounts except Inventory). When I run the R42800, it fails with error message in WC: Account number invalid. It concatenates the F4201.MCU (Branch plant) with the Object account of the AAI RC while I expect it combines F4211.EMCU ( BU entered manually in the sale order row) + Object account of AAI RC

 Do you know what's wrong in my setup?

Many thanks in advance for your help :-)