How had you loaded the PO itself?
The RM06EEI1 is in general a subsequent step to RM06EEI0, whose docu says: "By means of the appropriate organizational measures, please ensure that the POs created via program RM06EEI0 are not changed manually before program RM06EEI1 has been run successfully. Inconsistencies cannot otherwise be ruled out"
And the docu to RM06EEI1 says that you should not have schedule lines if you load history.