I upgraded (by accident!) to version 3 today and noticed a few warnings about undefined variables. I fixed it by adding this line at the top of
function at_build_panels_layout_css($panel_styles, $device_panels_data), around line 70 in adaptivetheme/at_core/inc/forms/at_core.submit.builders.inc:
$declarations = $these_groups = $these_selectors_styles = array();
I'm actually using the footheme, which I guess works best with version 2, but like I said, it was an accident. So this might not be an actual bug, just an artefact of my install.