Connection lost. Saving has been disabled…

The full message I’m receiving is “Connection lost. Saving has been disabled until you’re reconnected. We’re backing up this post in your browser, just in case.”

It doesn’t matter how simple the change I’m making to the page is, or what browser I’m using – almost every time I try to make a change to one particular page of our website I get this error and I have to retry multiple times before my changes are actually saved. It does happen to be one of the largest pages of our website.

I started to experience this problem once upgrading to WordPress 4.0 – I also installed some security plug ins at the time but have since tried disabling them to see if they were the problem. It didn’t help. I also tried to reinstall WordPress 4.0 and that didn’t help either.

Please help and thank you in advance.