Whats wrong with this circuit?
It's funny, I teach EE and I recently gave a circuit quite similar to this to my student. The problem here is all about matching impedance. Because of the inverting amp, you have a virtual ground on the negative port of the op-amp. That virtual ground is the root cause of your impedance problem along with closely matched resistors.

With a small redrawing you can create a small model of what is going on: 17k in series with two 1k resistor in parallel that forms a voltage divider. Thus, you're input impedance of your op-amp is greatly reduced.

Vout = 1k//1k / (1k//1k + 17k) * 15 = 0.5 / (0.5 + 17) * 15 = 428.57 mV

To correct this you can either A) put a voltage follower between the voltage divider and the 1k resistor of the summing amp or B) increase all the 1k resistor of the summing amp to a much greater value. E.G: 30k.

30k // 1k = 960 ohm for the lower equivalent resistor.
 0.96 / (0.96 + 17) * 15 = 801.7 mV

You could increase the summing amp resistors even more if you really want to push the theory to the limit. But, most often you try to keep feedback resistors in the 1k-100k range.