It happens to me every few weeks but not enough to concern me.
In case it matters, I use Opera 12.16 with Win 7.
The last time it happened was a day or so ago but I can't recall if it was just before or just after I uninstalled AdBlock Plus when it wouldn't whitelist duckduckgo.com.

I have noticed that every once in a while, a second cookie will show up on the Allow list in CCleaner for a site that's already there.
I assume that now and again a site updates cookies and, because they're not quite the same, they get listed, too, but being no kind of tech expert, that is seriously a guess and may have nothing to do with this at all.

Gil.

If my rememberer is working - not always a given - I think MaG said a security setting has you sign back in periodically. I also had to sign in a day or so ago, but I assume that was the security setting.

I had to uninstall Adblock Plus too... it wouldn't let me watch the video of a rescue of 11 starving horses we did two weeks ago. I tried to whitelist it, but it still wouldn't let me watch the video.

I was just going to post all has been well - and found I was logged out again At least I didn't have to go through the login loop again, so that's a step in the right direction.

If the IT guys say there's nothing unusual about the code, I'm going to assume it's a Microsoft enhancement security fix.

I've had to sign in twice today. I'm using IE 11 on Windows 7 64 bit. Draclvr mentioned Adblock Plus, so I disabled it and will see if it makes a difference.

I have AdBlock Plus, Windows 7, and Firefox, and have had no sign-in issues whatsoever.