The problem is NOT saving the saving the settings. The plugin clearly shows that the settings have been saved.
The problem is that you are not asking the right questions. You should be asking for the bucket, the regional server, and the folder containing my downloads. Also the plugin should not be trying to list every file in every folder in every bucket in every region on the planet earth.
Also, you should have some diagnostic routine to show what your plugin is doing.
See the screen shot below:
