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Thanks for your question, and thanks for including the text of your shortcode; very helpful.
When filtering a gallery on a taxonomy term the parameter name is the “slug” for the taxonomy in which the term(s) are assigned. If your term is in the WordPress Categories taxonomy, use category=blumen. If your term is in MLA’s Att. Categories, use attachment_category=blumen. That should get you the results you seek.
I am marking this topic resolved, but please update it if you have any problems or further questions regarding the above suggestions. Thanks for your interest in the plugin.