The problem was, that the bitcoin daemon is doing something during the first ~10 seconds upon start up. It's not downloading the blockchain, it did it before, it's something else. Nevertheless it returns 401 during this time. After ~10 seconds everything's fine.
I just don't understand why there are so negative posts, I'm learning bitcoin.
Yes the bitcoind is reading in the blockchain and verifying the last 200 blocks at a very high level to make sure your blockchain isn't corrupted since the last shutdown.
These aren't negative post, we just don't want you or anyone to start taking in funds without knowing what you are doing. We are just warning you.