The plug-in manager in the window is blank after opening, and no plug-in information is displayed. The network is connected.
Are you located in China? We have found that some (not all) Chinese ISPs block the server on which the Plugin Manager information is located. At least this is what we assume at this moment.
Plugin Manager will work in a different WiFi, or when you go online with your phone instead. Here how to get your Mac online with your phone (in Chinese language).
You can verify your internet yourself: This is the page that needs to be accessible:
If it opens (it has a lot of text), Plugin Manager will work. If it does not open, you need to connect in a different way.