Opera browser says this in developer console:

Blocked loading mixed active content “http://yui.yahooapis.com/combo?2.9.0/build/yuiloader-dom-event/yuiloader-dom-event.js&2.9.0/build/connection/connection-min.js”

I think that mixed content means that my website is https and it loaded http element (which is considered wrong).
Generally to fix this theoretically one needs to find the script in which is called the external mixed content and replace http by https or better remove http/https and replace it by //

in my case turn "http://yui" into "//yui"

But i have vBulletin CMS, so i used my CMS control panel. Inside vBulletin when one go to Styles & Templates / Search in Templates and search for "yui", he finds template file "headinclude"
inside is:
<vb:if condition="$show['remoteyui']">
<vb:if condition="$show['remoteyuicombo']">
<script type="text/javascript" src="{vb:stylevar yuicombopath}?{vb:stylevar yuiversion}/build/yuiloader-dom-event/yuiloader-dom-event.js&amp;{vb:stylevar yuiversion}/build/connection/connection-min.js"></script>
<vb:else />
<script type="text/javascript" src="{vb:stylevar yuipath}/yuiloader-dom-event/yuiloader-dom-event.js"></script>
So this code can be editted but i found that in vbulletin admin area / Settings / Server Settings and Optimization Options is the option to use custom URL for yui. one can set https instead of http. But in my case these outdated custom yui URLs not worked, so i deleted custom urls and set Google as a yui source. Now it delivers .js not from yahoo, but from google and via HTTPS which is what was needed to get rid of error "mixed active content": https://ajax.googleapis.com/ajax/libs/yui/2.9.0/build/yuiloader-dom-event/yuiloader-dom-event.js