Here is some more information about the problem.

It has something to do with the query-Parser and the property TIB_Query.KeyRelation

a Query with a Subquery and an empty Keyrelation like this

SELECT TXMANUMM,TXSPRACHE,TXTEXT,

  (SELECT SPNAME FROM SPRACHE WHERE SPRACHE.SPNUMM=TEXTEBEST.TXSPRACHE) as SPRACHE

FROM TEXTEBEST

ORDER BY TXSPRACHE

works with FB2.5 when i try to insert a row but not with the FB3.0 server.