I've copied this post from an existing one in the b2evolution support forum.
The reason is that now the Calendar is a plugin it probably would be more at home here, but there was an existing thread in the support forum.
The background is that I was getting HTTP 403 (Forbidden) errors when clicking on archive links in the calendar. I'm not sure why, but this only happened when they had a traling slash, eg.
Remove the trailing slash and everything seems fine.
It seems like for some reason I'm the only one having this problem, but just in case it's happening to anyone else is suffering from it too, I've patched my install of 1.9.3.
Since the calendar is now a plugin rather than function, the code you need to edit has moved but it's the same principle.
I replaced one line (I think it was 770) in /plugins/_calendar.plugin.php from something like:
echo '<a href="'.$this->archive_link( $this->year, $this->month, date('d',$i), '' )'"';
$link = $this->archive_link( $this->year, $this->month, date('d',$i), '' );
echo '<a href="'.substr($link,0,strlen($link)-1).'"';