Setting a parent COU on an already set COU throws an error:

SQLSTATE[42883]: Undefined function: 7 ERROR: operator does not exist: integer >=
LINE 1: ... AS "Cou__id" FROM "cm_cous" AS "Cou" WHERE "Cou"."lft" >=
^
HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts.

This is because TreeBehavior::_sync is trying to pass the following condition to updateAll:

Array
(
[0] => Cou.lft >=
)

and Postgres doesn't allow "where foo >= NULL". (MySQL does, and returns no matching rows.)