We fixed this error in SAP Connector v6.3.2:
"A degrade occurring only in release 6.3 could cause a very limited number of SQL statements to fail (for example a statement containing JOIN in this Jira issue). The error is now resolved" (From release notes for v6.3.2)
So upgrading to version 6.3.2 should resolve your issue.
// Thomas Örnmarker
Indeed, it did solve my situation in which the same error occurred while trying to execute a simple SELECT ListOfAllFields FROM KNA1; No JOIN involved, and the error seems SAP system-dependent as I was trying to extract the same table simultaneously from multiple SAP systems. Only one failed.
Thanks for the solution.