Lots of errors in event log: WordPress database error Unknown character set

If you need a temporary fix to use the plugin while you are waiting for the author to provide a permanent solution, you can modify this line of code in the calendar.php. It’s most likely that your MySQL server setup doesn’t handle utf8mb4 charsets.

Lines 775-781

// We've installed/upgraded now, just need to check the charsets
    $charset = $wpdb->get_results("select table_collation from information_schema.tables where table_name like '" . WP_CALENDAR_TABLE . "%' and table_collation not like 'utf8mb4%'");
    if (!empty($charset)) {
        $wpdb->get_results("ALTER TABLE " . WP_CALENDAR_TABLE . " CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci");
        $wpdb->get_results("ALTER TABLE " . WP_CALENDAR_CONFIG_TABLE . " CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci");
        $wpdb->get_results("ALTER TABLE " . WP_CALENDAR_CATEGORIES_TABLE . " CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci");
    }

change to…

// We've installed/upgraded now, just need to check the charsets
    $charset = $wpdb->get_results("select table_collation from information_schema.tables where table_name like '" . WP_CALENDAR_TABLE . "%' and table_collation not like 'utf8%'");
    if (!empty($charset)) {
        $wpdb->get_results("ALTER TABLE " . WP_CALENDAR_TABLE . " CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci");
        $wpdb->get_results("ALTER TABLE " . WP_CALENDAR_CONFIG_TABLE . " CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci");
        $wpdb->get_results("ALTER TABLE " . WP_CALENDAR_CATEGORIES_TABLE . " CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci");
    }