How to fix nginx throws 400 bad request headers on any header testing tools?

As stated by Maxim Dounin in the comments above:

When nginx returns 400 (Bad Request) it will log the reason into error
log, at “info” level. Hence an obvious way to find out what’s going on
is to configure
error_log
to log messages at “info” level and take a look into error log when
testing.

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