A problem that is happening on your site, we went to test how this can be reproduced
date.php
line:123
} elseif ( is_month() ) {
$one_month = query_posts( "posts_per_page=-1&year=$ye&monthnum=$mo" );
$output = raindrops_month_list( $one_month, $ye, $mo );
I can reproduced when $one_month value is empty array()
$one_month = query_posts( "posts_per_page=-1&year=$ye&monthnum=$mo" );
The return value of this function is, for the core function, can not be changed
Please try alternative function
If possible, Try change from query_posts to get_posts like below
date.php line:123
$one_month = get_posts( "posts_per_page=-1&year=$ye&monthnum=$mo" );