Hi,
I had the same issue a few days ago, just in case anyone faces this issue and looks for help in this forum...
Try checking bank accounts master data in FI12 (table T012K), every time you define the alternative account number in BNKN2 field, the program will ignore the account number defined in BANKN.
So you will need to map the account number sent by the bank against BANKN (if BNKN2 empty) or you will need to map it against BNKN2
Regards,
mcc