You find an SAP note which seems to describe the same problem you met. And in the "Solution", it documents something like following:
e.g: note:1911087 - DB6: DROP INDEX to DROP CONSTRAINT
Use an up-to-date version of the database interface library for IBM DB2 Linux, UNIX, and Windows.
patch text: "DB6: DROP INDEX to DROP CONSTRAINT"
So you want to check if this patch has been installed or not.
SAP on IBM DB2 for Linux, UNIX, and Windows
KBA , BC-DB-DB6 , DB2 Universal Database for Unix / NT , How To
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