All the Ingres error codes have funky looking (hex) numbers preceded
by E_, W_, or I_  and a two-letter facility code.  E.g. E_QE0002 is a
generic query execution code.

Embedded SQL error codes are different but they are generally largish
negative numbers like -38000.

I don't know any error code -6.  I suspect that your application is trapping
some Ingres error and translating it to -6.  You'll have to find out
what Ingres error code the application is getting.

The closest that I know of to an error listing is the file
$II_SYSTEM/ingres/files/english/messages/messages.txt.  I am not
entirely sure how it's generated (automatically, one would hope), and it
may contain additional messages that front-end tools like qbf can emit,
along with DBMS server messages.

Karl