I had this last week, it was suggested that I check the date/time on my computer. So even though it appeared correct, I changed the time to be the same time it said and saved that. I have not had the security message since then.
I used to have it with Chrome a lot. Oddly enough it hasn't happened lately. It was annoying though. I found if I cleared my cache and waited about 30 minutes before accessing my activity feed or convos again - the wait is important - then I was able to access the feed and convos without a problem again. It happened periodically but that worked to fix it for a few days at a time.