ltree_plpython failure test on Cygwin for 12.4 test

I am finally trying to move from python2.7 to python 3.x

for both 3.7 and 3.8 I have (attached log):

2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython
ERROR: incompatible library "/pub/devel/postgresql/prov
a38/postgresql-12.4-1.x86_64/build/tmp_install/usr/lib/postgresql/plpython3.dll":
missing magic block
2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython
HINT: Extension libraries are required to use the PG_MO
DULE_MAGIC macro.
2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython
STATEMENT: CREATE EXTENSION ltree_plpython3u CASCADE;
2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython
ERROR: language "plpython3u" does not exist
2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython
HINT: Use CREATE EXTENSION to load the language into th
e database.
2020-08-12 18:35:47.433 CEST [10418] pg_regress/python3/ltree_plpython
STATEMENT: CREATE FUNCTION test1(val ltree) RETURNS int
LANGUAGE plpython3u
TRANSFORM FOR TYPE ltree
AS $$
plpy.info(repr(val))
return len(val)
$$;

Only the python tests fail

$ grep FAIL postgresql-12.4-1-check.log
test python3/hstore_plpython ... FAILED 423 ms
test python3/jsonb_plpython ... FAILED 172 ms
test python3/ltree_plpython ... FAILED 163 ms

never had problem with python2.7

Suggestion ?

Regards
Marco