Using file as the backend, I had to set this in WikiUsernew.php to be able to generate an automatic homepage.
$page = $request->_dbi->getPage($this->_userid);
$page->save("Automatically created users homepage to be able to store UserPreferences.", 1, $versiondata);
$request->_dbi->touch();
$this->_HomePagehandle = $request->_dbi->getPage($this->_userid);
Logged In: YES
user_id=13755
Originator: NO
This solution is too specific. Prefs are not dependent on database backend types. We will find a better solution.