I had this problem while using QT 5.6, Anaconda 4.3.23, python 3.5.2 and pyinstaller 3.3. I had created a python program with an interface developed using QTcreator, but had to deploy it to other computers, therefore I needed to make an executable, using pyinstaller.
I've found that the problem was solved on my computer if I set the following environment variables:
QT_QPA_PLATFORM_PLUGIN_PATH: %QTDIR%\plugins\platforms\
QTDIR: C:\Miniconda3\pkgs\qt-5.6.2-vc14_3\Library
But this solution only worked on my PC that had conda and qt installed in those folders.
To solve this and make the executable work on any computer, I've had to edit the ".spec" (file first generated by pyinstaller) to include the following line:
datas=[( 'C:\Miniconda3\pkgs\qt-5.6.2-vc14_3\Library\plugins\platforms*.dll', 'platforms' ),]
This solution is based on the answers of Jim G. and CrippledTable