nginx – Incomplete response received from application when character not ascii in HTML format

I have an 18.04 Ubuntu server with Nginx and Phusion Passenger (non-Rails application) and it only works for websites with ASCII characters in HTML.

But when I put a non-ASCII character to the index.erb as a, I receive an error:
Incomplete response received from the application

my place is:

LANG = C.UTF-8
LANGUAGE = C.UTF-8
LC_CTYPE = "C.UTF-8"
LC_NUMERIC = "C.UTF-8"
LC_TIME = "C.UTF-8"
LC_COLLATE = "C.UTF-8"
LC_MONETARY = "C.UTF-8"
LC_MESSAGES = "C.UTF-8"
LC_PAPER = "C.UTF-8"
LC_NAME = "C.UTF-8"
LC_ADDRESS = "C.UTF-8"
LC_TELEPHONE = "C.UTF-8"
LC_MEASUREMENT = "C.UTF-8"
LC_IDENTIFICATION = "C.UTF-8"
LC_ALL = C.UTF-8

All files are in UTF-8 as well of course.

Ideas? I am already out of myself ..

Thank you