[Plugin: Advanced WQSF] can't add taxonomies or custom meta fields

Hi! I’m a student and totally inexperienced in working with wordpress. For my webside I want to create a search form with the plugin “Advanced WP Query Search Filter”. For that I try to add a taxonomy or a Custom Meta Field. When I finish working on that and press “add taxonomy”, the screen gets white and doesn’t do anything.
(It’s still possible to add new search forms).
Would be nice, if you could explain me what I’m doing wrong!