The warning was an empty <span> tag. I found it in [\inc\_core\ui\forms\_form.class.php Line 174. Remove the value and just left '', an empty string.
Line 173: 'title_fmt' => '<span style="float:right"> $global_icons$</span><h2>$title$'."\n",
Line 174: 'no_title_fmt' => ''."\n",
I've had to put a space before '$global_icons$' above to make the text wrap to the container.