Issue was solved by the poster, see the copied comment below. Added this here to make sure the issue is marked as answered.
Thanks for asking, I just found the mistake though. The problem was that the JSON, that the REST Service offered, looked slightly different than the JSON, that was fed to it (It's my own API). So in my JSON Structure I had the structure of the "original" JSON, while the REST Service slightly changed the structure of the outside brackets.