Cookies problem

I paused cloudFlare but that didn't solve the problem.

Sometimes when I visit website pages I get a blank page with error message

400 Bad Request
ـــــــــــــــــــــــــ
nginx

when CloudFlare is enabled the error is
400 Bad Request
cloudflare-nginx

I searched the web for this error I found "400 Bad Request Request Header Or Cookie Too Large nginx", the session cookies file is not too large is is only 1KB.
I tried clearing cookies cache, but that didn't solve the problem.

I wonder does the internet provider has effect on cookies !!!
it is annoying problem.