This sounds like your Pi might be running the 4.9 Linux kernel, with which webiopi (a component of the Cayenne Pi agent software) is currently incompatible. Could you run the command
uname -a to check your kernel version?
If it is showing 4.9, you can switch to the 4.4 kernel with just one command (and a reboot afterwards):
sudo rpi-update 52241088c1da59a359110d39c1875cda56496764
We have an update to the Pi agent in our QA now which should resolve this issue and allow Cayenne to work on both 4.4 and 4.9 in the near future.
uname -a shows you are already running 4.4, let me know and I'll continue to help troubleshoot.