Could not load firmware file zd1211/zd1211b_ub. Error number -2

To solve this issue you need just to install the zd1211 firmware:

sudo apt-get install zd1211-firmware

If you are using Ubuntu or debian you need to add "non-free" repo, for example for Debian 8 you need to add the following source:

deb http://http.debian.net/debian/ jessie main contrib non-free

Then you will be ready to install zd1211-firmware.