There appears to be a bug in the WiFi101 library that affects the ThingSpeak library and examples. After a few transactions, the library stops working.
Until the bug gets fixed by Arduino, you can use version 0.13.0 of the WiFi101 library.
To change the library version, open up the Arduino IDE and go to Sketch > Include Library > Manage Libraries...
Then search for WiFi101 and click "More info" under the WiFi101 section. Select "version 0.13.0" and click Install.
After the library is done installing, you will need to recompile your project.