Hi Rahul,
Your error information is not in detail, so this is just my guess.
There is a HANA bug, MultiColumnJoin fails with UNKNOWN_ATTRIBUTE if parallel CREATE/DROP concat index.
concat attr was deleted from memory during DDL call, so old readers (transactionally still seeing the concat) failed to read attribute data.
As I know this bug will be fixed in rev102, so I suggest rev102 if it is possible for u.
Best regards,
Candy