Libraries/Calendar.php, Line 163, replace A with B. (This is the same as Geert’s first posted opt, so should be good)
A :
// Add a trailing slash to the URL if needed
$this->next_prev_url = preg_replace("/(.+?)\/*$/", "\\1/", $this->next_prev_url);
B :
// Add a trailing slash to the URL if needed
$this->next_prev_url = rtrim($this->next_prev_url, '/') .'/';
