Go step by step and test.
1. Log out of your account.
Click menu in the menu bar and choose “Shut Down”.
Press the power button to start the Mac.
2. Launch System Preferences.
Look at the bottom of the pane.
Do you see Trusteer Rapport or Tusteer Endpoint installed?
If yes, click it and stop it
or Uninstall Trusteer Rapport/Endpoint.
https://www.trusteer.com/support/uninstalling-rapport-mac-os-x#1
3. Do you have Airfoil app installed?
4. Deselect Proxies, if selected.
System Preference > Network > Advanced > Proxies Tab
Unlock the lock if you have to.
Under "Select Protocol", uncheck any box if check marked.
Click "OK" then "Apply”.
5. Sometimes third party software can cause this kind of a problem.
Please run EtreCheck and post the report here.
https://etrecheck.com
Download it, open Downloads folder, click on it to open and select ”Open”.
Select a problem from the “Choose a problem” popup menu box, and then “Start EtreCheck” in the dialog.
Click “Share Report” button in the toolbar, select “Copy to Clipboard”, and then paste it when you reply.