I am using selfoss 2.16 and the standard "RSS feed" spout.

In which way should I change the helpers/WebClient.php?

Replace

return $userAgent.' ('.implode('; ', $agentInfo).')';

with something like

return "Mozilla/5.0 (X11; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0";

(the user agent of a browser, that displays the feed correctly)