I've not gotten this, however, I often find that it has to do with browser alert settings. Browsers check sites for their security - this has security which you can see in the url with the https: that 's' is the clue. But to have security there are a lot of things that all have to work together. Images have to be in a safe place, links have to be valid and not to some virus laden ad, etc. Even the owner of the certificate has to have a valid name and address. So if your browser is looking at all of that and finding something off, it may alert you about the site. And it is looking every time you come to the board.
To resolve it in this case, you need to make sure your browser is up to date, and then you can add the website to your 'safe' list in your browser site list so it stops rechecking the security.
If it isn't this, then you really want to run a virus check on your computer while NOT being connected to the Internet and see if there is a bug in your browser. There are several non-destructive viruses that just hijack your browser. Non-destructive in that they don't hurt your computer, but they grab anything you type in and watch where you're going and so forth. All not good.
Hope that wasn't too much geek speak, but I'm a programmer analyst by trade, and I own an Internet based company in which I am the web tech. So I have these issues with my users from time to time and have to walk them through it all.
Happy computing!