I understand that this is unfortunate, but I doubt that you want the semantically correct query:

update NACHRICHT 
set titel='abc' 
where LID=(
  select N.LID 
  from NACHRICHT N 
  join PORTALNUTZER PN on N.LSENDERID=PN.LID 
  where PN.LID=?
)

You will just have to switch to the fk function if you want to access the foreign key column of an association that has a NotFoundAction specified.