Being familiar with German traffic laws as a driver and given that barbells are not illegal or anything else, the only constraint imposed upon you by the law and regulations is that your luggage be safely stored! Safely stored as in does not pose a threat to you or other people even in case of an accident and it should not impede your ability to operate the vehicle. That includes not obstructing your sight through the mirrors and of course your vehicle should not be overloaded either.
Here is a great instructional video of German automobile club ADAC that gets the message across even if you don't understand German. The big umbrella in the video could be compared to the barbell I guess. (There are some tips on how to safely store your stuff on this related page, in German, or here for skies, again in German.)
E.g. the barbell should not be able to slide through the front windshield if you were to brake heavily. Judging from your picture you are good on that one.
Safe and overloaded to some extent are judgemental calls, with the ultimate call given to a police officer should they decide to stop you. That being said, if you use a handful of common sense you should be fine.