Not a real fix, but at least a workaround:
add the following code to your theme functions:
remove_filter('widget_text_content', 'wpautop');
remove_filter('widget_text_content', 'wptexturize');
remove_filter('widget_text_content', 'capital_P_dangit');
remove_filter('widget_text_content', 'convert_smilies');
this will give you the old behaviour.
Source:
Addition of TinyMCE to the Text Widget
