This error shows up for many people and might have many potential reasons. In my case I started getting that error after using a Firefox add-on to unblock some sites usually not accessible in my country by going through a proxy. Symptom has been that I was able to visit this site but started to get this error typically for sites in foreign countries ( not for sites in Germany ). The problem shows up only when using Firefox of course, not when using other browsers like Chrome or Internet Explorer.
Even after disabling that add-on & re-starting Firefox that problem didn’t go away. After a while I figured out that this add-on had changed my proxy settings ( that’s basically how it works ) but failed to set it back to normal.
After heading to Tools –> Options –> Advanced –> Network –> Connection Settings and changing my settings back to “No proxy” the problem was solved.