That is the most bizarre error i ever seen! What website were you at before you went to mixer? Were you following a link to mixer from another website? Before you even clicked on the link from that website did it show mixer.com on the bottom left like some browers do? If you have no clue what i am asking a lot of modern browsers have a hidden task bar on the bottom so when you move your mouse over a link it shows the URL of the link. I use this and love that feature cause you can almost clearly see something like mixer.com as a clickable link but you can make sure it actually is going to mixer.com and not mixerdotcom.somehackerwebsite.com/mixer.com.html
I am no coder and know nothing about modern web design and what they do for security but how i interpreted on what is going on is you were at some website that had a link to some persons mixer page. it quickly went to the fake site to activate a tracker and when it redirected again to the actual website ,some back end mechanism kicked in with this "safe error" where it really means "some hacker tried to steal your info we cut it off so no need to worry. please visit our website the normal way to continue properly"
That may sound scary and probaly not the case but again i have no clue what you were doing prior to visiting mixer.com
If you are embarassed and not tell us you were at some weird porno site before you wanted to watch a live stream we get that but somehow the mixer website did not like what website you were at before visiting. Probally @staff or @admin can better explain this. I also suggest you open a ticket at http://mixer.com/contact where you can get better help then from some random dude like me with a weird way of thinking why you may have a problem like this.