Step 1.

I upgraded the NGINX version from 1.18 to 1.26.2 and then encountered an error.

upstream sent invalid header: "\x20..." while reading response header from upstream

Step 2.

I checked the NGINX CHANGE LOG and found that version 1.21.1 had relevant modifications.

Changes with nginx 1.21.1                                        06 Jul 2021

    *) Change: now nginx always returns an error for the CONNECT method.

    *) Change: now nginx always returns an error if both "Content-Length"
       and "Transfer-Encoding" header lines are present in the request.

--> *) Change: now nginx always returns an error if spaces or control
       characters are used in the request line.

--> *) Change: now nginx always returns an error if spaces or control
       characters are used in a header name.

--> *) Change: now nginx always returns an error if spaces or control
       characters are used in the "Host" request header line.

    *) Change: optimization of configuration testing when using many
       listening sockets.

    *) Bugfix: nginx did not escape """, "<", ">", "\", "^", "`", "{", "|",
       and "}" characters when proxying with changed URI.

    *) Bugfix: SSL variables might be empty when used in logs; the bug had
       appeared in 1.19.5.

    *) Bugfix: keepalive connections with gRPC backends might not be closed
       after receiving a GOAWAY frame.

    *) Bugfix: reduced memory consumption for long-lived requests when
       proxying with more than 64 buffers.

Step 3.

Then I used CURL to access the URL and found that the response header had errors.

johnny$ curl -I -k -L "https://10.10.223.223/service"
...
...
Strict-Transport-Security: max-age=31536000;       <---
        includeSubDomains                          <--- It should be one line.
...
...

REFERENCE