This error is a result of how the iSeries ODBC driver handles return strings. It requires 1 additional byte beyond the defined string length.
1) Use the previous version IBM Client Access ODBC driver instead of the IBM iSeries Access ODBC driver.
2) Temporarily change all char and varchar stored procedure parameters. Increase their size each by 1 byte then re-apply the RSSql data object. Then change the stored procedure parameters back to their original size.
3) Use another database to proxy the data between RSSql and DB2 (like MSSQL Server distributed database transaction).