What is the group ownership for the files you create as user xxx? If it's not www-data, then the server won't be able to read them; you need to chgrp www-data filename … them.

Note that having your web directories/files writeable by the web server is a potential security risk.

You could add user www-data to group xxx and set all the files under foobar.com/htdocs to be owned by xxx:xxx. You'd have to restart lighttpd to get the group permissions working. That would give your xxx user the ability to edit their content and give lighttpd access to the content through the xxx group.