Originally Posted by Manichean
From your custom column being called "gelesen" I'm assuming you're using the german translation of calibre? If so, you'll have to use "#gelesen:Ja" or "#gelesen:Nein" to get any meaningful results. I recently ran into that same problem trying to setup a library restriction between study textbooks and, well, anything else.
I use English, but be aware there's a difference between searching for True/False versus Yes/No. True means there's an entry there (it could be Yes or No). False means the opposite. True and False are defined in Python. AFAIK, Yes/No or Ja/Nein are not. I don't know if you need to translate True/False, but I suspect you don't.