Everything looks fine. The transaction you showed confirmed ages ago, and the balance is reflected in your account, from what I can see in your screenshot.

I don't use bitcoin.de myself, but I understand it's a P2P marketplace just like Localbitcoins, which I use. If we assume they work similarly, your entire Bitcoin has already been reserved in a sell order - therefore, it's marked as a Reserved Balance. This is to ensure that you can never sell more Bitcoins than you actually have and you can fulfill all orders.

You should look for your order history details. You probably already put up a Sell Order for that amount or agreed to a Buy Order from someone else. I have a feeling you already put up a Sell Order. The only way for your Bitcoin to be released into available balance (so you can attempt to sell it again) is to have that existing order cancelled.

So... just cancel your Sell Order.