As you can noted in the README of the Ubidots Ethernet Library we metioned the following note:
“IMPORTANT NOTE: The library was tested using a shield with W5100 chip Ethernet controller.” In order of this, if you are using another chip we can ensure the right functionality of it.
I recommend you find a WebClient example (which is easily to find on internet) for the Arduino Leonardo, make a test with the sample code to ensure the righ functionality of it. Then, once the first example code works properly modify the request establish in the sample to the Ubidots server.
For more information about the Ubidots REST API Reference, please see here.
All the best,