cx-freeze "ImportError: DLL load failed: %1 is not a valid Win32 application"

I am getting an error after I install a python/pyqt/matplotlib app that I freeze with cx-freeze. I build the app with

python setup.py build

setup.py is this

from cx_Freeze import setup, Executable
buildOptions = dict(packages = [], excludes = [])
import sys
base = 'Win32GUI' if sys.platform=='win32' else None
executables = [
    Executable('SectionPropertyCalculator.py', base=base)
]
setup(name='Mecanica-SectionPropertyCalculator',
      version = '0.1',
      description = 'SPC is a GUI to calculate geometrical properties of beam profiles.',
      options = dict(build_exe = buildOptions),
      executables = executables)

I get

Mecanica-SectionPropertyCalculator-0.1-amd64.exe 

The .exe runs ok on my dev machine

Then I create a msi installer with

cx-freeze app bdist_msi

I get

Mecanica-SectionPropertyCalculator-0.1-amd64.msi 

I install it into another machine with same OS, i5 processor, etc. and I get an error

File "C:\...SpcPlotQt.py", line 4 in <module> 
ImportError: DLL load failed: %1 is not a valid Win32 application

Now, SpcPlotQt.py is my code and on line 4 I have

from PyQt5.QtWidgets import QDialog, QApplication

I have PyQt5.5.13.2 installed with pip3 (everything 64 bit) and I am pretty sure I installed python 3.7 x64, which I confirm by

import struct; print( 8 * struct.calcsize("P"))
64

Also, if I run

import sys; print("%x" % sys.maxsize, sys.maxsize > 2**32)
True

and

import ctypes; print (ctypes.sizeof(ctypes.c_voidp))
8

and

import platform; platform.architecture()[0]
64bit

and

import os; os.environ["PROCESSOR_ARCHITECTURE"]
AMD64

All 5 tests above tells that everything is 64 bit.

But when I run

import sys; print (sys.platform)
win32

Why?

AND... cx-freeze scripts use sys.platform to decide what base to use, thus it chooses

base=Win32GUI

What should I do?

I notice that python setup.py build runs out of a cmd shell, which is always 32 bit, no matter how I open it; even a PowerShell is 32 bit. Could this be the problem?