Hi Hansjakob Fehr,
I misunderstood in the previous replies. My replies above worked for WordPress's taxonomy box.
For your situation, as the conditional field is just a normal Meta Box field, you don't need complex conditions. Instead, you can just write:
'visible' => array (
'when' => array (
array('medium', '=', 2 ),
),
),
Where 2
is the ID of the term buch
. The taxonomy advanced field uses term IDs, so we have to use them instead of slugs.