If I use UPX, my app crashes as soon as it starts (there's no console output when I use cmd.exe). Weird thing is, bundled app with --onefile works with no issues (files seem to be compressed, *.exe is smaller when I use UPX if I recall correctly).
Everything works well in Windows 7 x86 too.

According to Windows 7 error message, "ucrtbase.dll" fails but according to Windows 10 (x64) error message, "vcruntime140.dll" does, so the workaround is easy: replacing each faulty *.dll file with its non-compressed version. It solves the problem but somewhat extensive testing is would be needed.
Have no tried Windows 8 yet.

OS: Windows 7 x64
UPX: 3.95 (x64 and x86), 3.94 and 3.93
Python: 3.7.3 x86 fresh install
Pyinstaller: 3.4 and 3.5dev0+b54a15d72e (no cache files)
Test toy code:

import os

print("hola")
os.system("pause")

A "hello world" app works though.

I'm new to GitHub so if there's something else I should do or test just tell me.
And thank you very very much for your hard work on PyInstaller!