UNION in SQL Server

[Anakim]

Here's some code which works like a watch with no hands

SELECT c.member_no, c.mo_ord_no
     ,c.cu_name --,'Nads' 
     , oh.ord_date
     , oh.ord_date

 FROM mcodcld c, mcodooh oh 
  WHERE c.mo_ord_no = oh.mo_ord_no
union all
SELECT c.member_no, c.mo_ord_no
     , c.cu_name, --'Nads' 
     , bh.ord_date
     , bh.ord_date

  FROM mcodcld c, mcodboh bh 
  WHERE c.mo_ord_no = bh.mo_ord_no 
union all
SELECT c.member_no, c.mo_ord_no
     , c.cu_name --,'Nads'
     , dh.ord_date
     , dh.desp_date
  FROM mcodcld c, mcoddoh dh 
  WHERE c.mo_ord_no = dh.mo_ord_no

I get a message about not being able to start another thread. Now if I comment out any one of the 'c.cu_name' and substitute in the fixed string of 'Nads' it works. Any pair combination also work without any amendments. I'm stumped on this one, can anybody shed any light?