Taxonomies exists – but not showing up

I think your problem lies with the use of get_object_taxonomies( $object, $output ). If you have a look at the codex, the $object parameter should be

Name of the post type, or a post object (row from posts)

As your code currently stands, you are using the taxonomy name, not post type. If you print $customPostTaxonomies with

print_r( $customPostTaxonomies);

you get Array(), that is why you get no output.

When you change get_object_taxonomies('jobfelder'); to get_object_taxonomies('jobboerse');, your code works and output the names of the taxonomies which is related to your custom post type ‘jobboerse’

EDIT

Thanks to @cfx

Better be safe than sorry when registering custom taxonomies for custom post types. Use register_taxonomy_for_object_type() right after the function (emphasis added) to interconnect them.