I thought I would share a fix I needed to do. I had a year in which I only posted in January. Because it means that the last month processed in the previous year was the same as the first month processed in that year (both cases, the month was 1), the widget failed.
My change was to insert a line at collapsArchList.php:113:
if( $currentYear != $archPost->year ) {
$lastYear=$currentYear;
$currentYear = $archPost->year;
$currentMonth = 0; # MSB, just to guarantee it won't match
$newYear = true;