This calendar is a very appreciated feature of our editors. And it runs without any problems since over a year.
But when trying to reduce the unnecessary log messages with WP_DEBUG enabled I found some messages from your plugin, too. One notice is
PHP Notice: Undefined variable: theme in /usr/www/xxxxx/wp-content/plugins/schedule-posts-calendar/schedule-posts-calendar.php on line 76.
The other is an outdated use of numeric capabilities (user_level) in the call of
add_options_page(). The backtrace displays
add_submenu_page(options-general.php,Schedule Posts Calendar,Schedule Posts Calendar,9,schedule-posts-calendar.php,schedule_posts_calendar_admin_page)
which is probably a subsequent call from
The user_level=9 is the problem in this call. You may simply replace it with a capability string from Roles and Capabilities wich reflects the admin equivalent to configure the calendar (e.g. 'edit_theme_options').