I found an error in the plugin updater that isn't that important, just annoying. When you install a plugin and an error occures during installation (syntax or any other error in source) the plugin ist installed nervertheless.
When you try to install it then in a error free version just the error occures:
AttributeError: 'module' object has no attribute
You must deinstall the plugin and restart Calibre. Afterwards everything is fine again.