I have just installed a new MacBook Pro and QGIS, everything is fine but this strange error if I try updating or reinstalling the "Processing" plugin.
The plugin is broken. Python said: cannot import name AlgorithmDecorator
The full error log is at https://gist.github.com/giacecco/43c6b206239bfaa16f97 .
This appears not to be a duplicate, although many other issues are on Stackexchange regarding this plugin.
Uninstalling also fails. No error messages are shown, but the plugin still shows in the "Installed" pane.