MB01 GR error Enter only one true account assignment KI249

Hi Experts,

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.

Thanks

Rijil