Check Safari > Preferenes > Extensions. If there are any installed, turn that OFF, quit and relaunch Safari to test.
If it's not an extensions issue, go to Safari > Preferences. Select the Security tab.
Deselect: Enable plugins. Quit and relaunch Safari. If you don't see the, "can't find server" dialog, follow the instructions here > Safari: Unsupported third-party add-ons may cause Safari to unexpectedly quit or have performance issues