Webpages not loading on one device but loads on another. How to fix this?

As it happens: none of the stackexchange sites open on my laptop, but they work fine on my tablet.

I have tried the following:

1. Cleared all cache cookies browser data. Didn't work.
2. Used another browser. Didn't work.
3. Tried browsing in incognito mode. Didn't work.
4. Disabled add-ons and extensions. Didn't work.
5. Restarted computer. Didn't work.

From these my diagnosis is that it is the device and not the browser. So here are facts about my device:

1. I am on ubuntu 14.04. I have installed a lot of things over time. But I don't remember installing anything harmful. Could it be because I haven't installed something essential yet? On chromium I use pepper flash. Firefox is unaltered since installation.
2. Sometimes the stackexchange websites do load. But most of the time it would not. Could it be some malware?

p.s.: the tablet and the laptop use the same network. So its not the network. And I don't think my network admin would block stackexchange on my laptop!