We are getting an error message as "Enter only one true account assignment KI 249" in MB01 transaction while doing goods receipt for a subcontracting PO. Subcontracting PO is created from a maintanence order for external operation. Account assignment of PO is with maintanence order. Account assignment F and item category L. Maintanence order inturn is reference to a sales order. We are into ECC 6 DIMP system. Same scenario is working fine in ECC 5 system and this issue is appearing in upgrade. Please provide your inputs for this problem.