I cannot change the proxy settings in Google Drive when connected directly. The Apply Changes button is greyed out

This can happen if you switched between "direct connection" and "automatically detect proxy settings". In my case, it was stuck on "direct connection", but at work I need to go via a proxy. This seems to be a bug in Google Drive. To "fix" it, open RegEdit and locate HKEY_CURRENT_USER\Software\Google\Drive and delete the "Direct Connection" key with value 1...