无法删除标题“vary：user agent”

I am trying to figure out why my Magento 2.2.6 shop is showing the header:

vary: Accept-Encoding,User-Agent

The problem is that the "User-Agent" makes my cache not working in a good way.

My shop is built like:

Visitor -> Nginx SSL Termination -> Varnish -> Apache 2.4

(This is under DirectAdmin with http2 activated.)

How can I find out where the Vary: User-Agent is added?

I have been looking in the Nginx config, Varnish default.vcl, and .htaccess

No luck, can you guide me where I should begin, and what I should look for?

(The problem is the same in production mode)

finally I found a forum post (in Polish), telling me in which file I can find the "vary" problem.

nano /usr/local/directadmin/custombuild/custom/ap2/conf/extra/httpd-ssl.conf

There I could remove the header. (Added by default by DirectAdmin)

Problem solved!