I haven't had problems with security, but I have yuku bookmarked and never sign in or out.
I did go look for an answer to the security notices and found that yuku is currently under beta testing for a transition to new tapatalk.
"The past few months also pointed us to the need to overhaul Yuku The Yuku code was written about 15 years ago (way before the first iPhone) and has been patched numerous times. We are at the point the continual patching is creating too many issues, and it is time to give Yuku the modern code it deserves.We are going to expand the partnership with Tapatalk to help us get to the next stage of Yuku. As part of the partnership, we will soon begin a transition to a rebuilt code base.
As many of you already know, a few months ago Yuku started a partnership with Tapatalk. Mobile traffic now account for over 50% of the traffic viewing Yuku forums. Given Tapatalk's expertise in mobile, it was a natural step in the evolution of Yuku.
As you also are aware, we have had many issues that impacted the availability of the forums. Initially, we had issues as we moved to cloud based servers (Microsoft Cloud and Amazon Web Services (AWS). Just recently, availability was impacted by a Denial of Service attack (DDOS). Working with Microsoft and Amazon, we have the attack controlled, and moved behind additional protection."
Those of you seeing a security warning are more than likely dealing with this "extra protection."
You might add tapatalk to your accepted sites and see if this doesn't resolve the problem. Let me know if it doesn't help.
I've now been added to beta testing and I think you are going to love the new forums. Adding links and photos are much easier. The posting boxes have been streamlined as well. I haven't had long to play with it yet, but it appears to be an upgrade!