Error when trying to update UserField in 9.0 PL11

Hi experts,

I'm currently tring to update userfield in the table ORDR,RDR1,OWTR and WTR1 using the DI API and each time an error occurs.

For the ORDR and OWTR table the error description is

Cannot drop the index 'dbo.OIEI.OIEIMRO_SO', because it does not exist or you do not have permission

For the RDR1 and WTR1 table the error description is

Cannot drop the index 'dbo.IEI1.IEI1MRO_SO', because it does not exist or you do not have permission

Do you know why this error occurs and why sap try to drop an index in the table OIEI or IEI1 when i'm updating a field in ORDR or RDR1?

Thanks.

Best regards