The output of ping says it's sending from 192.168.15.201, so that seems fine, just as the route.
Code:
#ip route show
default via 192.168.178.1 dev wlan0 proto static
192.168.15.0/24 dev usb0 proto kernel scope link src 192.168.15.201
192.168.178.0/24 dev wlan0 proto kernel scope link src 192.168.178.31 metric 2
/edit, what the hell? my routes are chaning. Now they are back to normal. And it does work.
/edit 2: Ok, it's all the fault of network-manager. I curse you!

