Okay, I don't know why I'm having such difficulty finding an answer online. I found an example of the sign on Google Images but the hosting site is down and there is no cache, and searches using "blue background red cross white arrow" aren't turning up relevant results. [Update, 2019: Now they do! This page being the top result.]
I'm driving for the first time in Europe—in Munich, Germany—and I came across an intersection where there seemed to a "main" signal above, straight ahead, and a "side" signal to the right, near the bicycle lane. The above pictured sign was affixed to its pole.
If this sign were standalone, there'd be less of a question about what it means. But...
It was a right turn from a smaller road to a main road, so it seemed unlikely that right turns wouldn't be allowed.
It was affixed to a side signal, so I thought it might mean to indicate that this side signal is a separate turn signal one should adhere to in order to make right turns (presumably to protect bicyclists and pedestrians).
A police car behind me, which I was nervously watching while waiting at the light, turned on its right-turn signal.
Was it fine to make a right turn? (The police didn't stop me, but I heard that you just get ticketed in the mail for violations.)