Hello Banal,
thanks for your help. After adding nickname.XML I found out that the problem came from the breadcrumbs. The breadcrumbs show the username when you view the user´s profile. I added an htmlentities() to ForumMemberProfile.php:
public function Breadcrumbs() {
$nonPageParts = array();
$parts = array();
$forumHolder = $this->ForumHolder();
$member = $this->Member();
$parts[] = "<a href=\"{$forumHolder->Link()}\">{$forumHolder->Title}</a>";
$nonPageParts[] = htmlentities($this->Title);
...
I did some frontend testing and didn´t find any bug caused by that change.