Reduced to simplest form, I ran into
a problem

SELECT A,
(SELECT Y.B FROM TABLE Y WHERE Y.B = X.A)
FROM TABLE X

Works fine .. but

SELECT DISTINCT A,
(SELECT Y.B FROM TABLE Y WHERE Y.B = X.A)
FROM TABLE X

produces following error

ISC ERROR CODE:335544343

ISC ERROR MESSAGE:
invalid request BLR at offset ..
context already in use (BLR error)

STATEMENT:
TIB_Cursor: "frmWISQL.crEdit"

Is this an Invalid Query or Bug ?

Best Regards
Hans