critical error

Hi, I am having this error when I try and block out times in my admin area, any idea of how to fix this please?

An error of type E_ERROR was caused in line 1640 of the file ../wp-content/plugins/appointment-hour-booking/cp-main-class.inc.php. Error message: Uncaught TypeError: Unsupported operand types: int + string in …/wp-content/plugins/appointment-hour-booking/cp-main-class.inc.php:1640
Stack trace:
#0 ../wp-content/plugins/appointment-hour-booking/cp-main-class.inc.php(1344): CP_AppBookingPlugin->extract_appointments(Array, Array, ‘_1’)
#1 ../wp-includes/class-wp-hook.php(308): CP_AppBookingPlugin->data_management(”)
#2 ../wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters(NULL, Array)
#3 ../wp-includes/plugin.php(517): WP_Hook->do_action(Array)
#4 ../wp-settings.php(623): do_action(‘init’)………