Calling a PL/SQL Function at the wrong time gets me into trouble – the dreaded ORA-0113 End of Comunication Channel with a hierarchical query

I have a query: an in line view, built from two simple queries unioned together, that forms the starting point for a hierarchical (connect by) query. Nothing up my sleeves:

with nodes as
( select bgg.id   id
  ,      bgg.naam node_label 
  ,      -1 parent_id
  ,      0 ctt_id
  from   pgi_v_begrotingen bgg
  and    bgg.id = 7837
  UNION ALL
  select bpt.id id 
  ,      pgi_util.TREE_CATALOGUSPOST_NUMMER_NAAM(bpt.ctt_id) node_label
  ,      nvl(bpt.bpt_id, bpt.bgg_id) parent_id   
  ,      bpt.ctt_id
  from   pgi_v_begrotingsposten bpt
  where  bgg_id = 7837
  )
  select node_label NodeLabel
  ,      level
  ,      id Id
  ,      parent_id ParentId
  ,      ctt_id CttId
  from   nodes
  connect
  by     prior id = parent_id
  start with parent_id =-1
  order
  siblings by   pgi_util.TREE_CATALOGUSPOST_sort(ctt_id)
 

yet it fails, depending on the environment, either with an ORA-03113: end-of-file on communication channel (SQL*Plus) or java.sql.SQLException) OALL8 is in an inconsistent state. (from ADF BC). 

It turns out that
the call to the PL/SQL function pgi_util.TREE_CATALOGUSPOST_NUMMER_NAAM(bpt.ctt_id) at that point in the query, causes the problems. When I rewrite the query – with help from Anton – to have the PL/SQL call made outside the hierarchical query:

with nodes as
( select bgg.id   id
  ,      bgg.naam node_label 
  ,      -1 parent_id
  ,      0 ctt_id
  from   pgi_v_begrotingen bgg
  and    bgg.id = 7837
  UNION ALL
  select bpt.id id 
  ,      pgi_util.TREE_CATALOGUSPOST_NUMMER_NAAM(bpt.ctt_id) node_label
  ,      nvl(bpt.bpt_id, bpt.bgg_id) parent_id   
  ,      bpt.ctt_id
  from   pgi_v_begrotingsposten bpt
  where  bgg_id = 7837
  )
, tree as  
(  select node_label NodeLabel
  ,      level lev
  ,      id Id
  ,      parent_id ParentId
  ,      ctt_id CttId
  from   nodes
  connect
  by     prior id = parent_id
  start with parent_id =-1
  order
  siblings by   pgi_util.TREE_CATALOGUSPOST_sort(ctt_id)
)
select case cttid
       when 0
       then nodelabel
       else pgi_util.TREE_CATALOGUSPOST_NUMMER_NAAM(cttid)
       end nodeLabel
,      lev  
,      id
,      parentId
,      cttId
from   tree
 

all of a sudden it does complete, as it should. Apparently, whatever is going on inside that function conflicts the query plan and execution picked by the CBO in the first attempt.