Hi László,
The plugin also caused this issue on my site, before I deleted it.
I suspect this is the problematic line:
<div data-position="below" class="n2-ss-widget n2-ss-widget-hide-mobileportrait nextend-shadow n2-ow" style="margin-top:0px;width:100%;"><img style="display: block; width:100%;max-width:none;" class="n2-ow nextend-shadow-image" data-no-lazy="1" data-hack="data-lazy-src" src="https://<mysite>/wp-content/plugins/smart-slider-3/Public/SmartSlider3/Widget/Shadow/ShadowImage/Assets/shadow/centered.png" alt="Shadow" /></div>
Cheers,
Anna