To be honest, it's quite easy to reproduce. See this:
Someone or something is attempting to "hack" or something on the board. In the code, you may try to find it at applications\calendar\modules\front\calendar\view.php:
/* Pagination */
$offset = isset( \IPS\Request::i()->offset ) ? min( array( \IPS\Request::i()->offset, \count( $events ) ) ) : 0;
As you can see, there is no check to ensure that the offset value is an integer. Please try changing it to:
/* Pagination */
$offset = isset( \IPS\Request::i()->offset ) ? \intval( \IPS\Request::i()->offset ) : 0;
$offset = $offset ? min( array( $offset, \count( $events ) ) ) : 0;
Should help.