The streetview image posted by advisedwang shows there's also a white-on-blue "must turn right" sign at that junction. This ought to make it clear that you shouldn't wait there to go straight.

Except that the traffic light takes precedence over the sign and thus the sign has no effect whatsoever as long as the traffic light is active.

No, it would take precedence if it had an arrow painted on it, but as it's just a traffic sign, it only controls entry into the intersection, not where you can go.

Tourists don't come that way. It's very small residential street, turning into a big street with lots of confusing signs and disallowed directions. For me the much worse problem is the other way, coming down the Güntzstraße to the Elbe. The whole crossing is a real city planning nightmare, and I worked in city planning for some years.

I think for most tourists the green arrow next to the red light should be suggestive enough.