installing gnome shell extensions timeout

I have Ubuntu installed on VirtualBox and behind proxy. I have set automatic proxy configuration. All applications are able to connect to internet.

However Gnome Shell extensions are not getting installed. With Ubuntu Software center, it shows a timeout error though other applications are getting installed. From Browser on https://extensions.gnome.org, on-off button seems to do nothing (browser extension and chrome-gnome-shell are installed).

With manual proxy setting, Ubuntu Software center throws GDbus.error:org.gnome.shell.downloadinfoerror:4

Can someone help me to fix this?

Note: sudo apt install <other apps> works without any error.