Maybe a BUG in the utils_wc_date_timeslots so that to calculate the timeslots it’s ESSENTIAL setting days of the week in the WorkingCalendar.
SELECT wc."TimeSlots"
FROM "WorkingCalendar" wc
LEFT JOIN "LookUp" priority ON priority."Id" = wc."Priority"
WHERE ( (REPLACE(wc."CalendarDate", year_pattern, date_part('year', reference_ts)::varchar))::date = reference_ts::date --specific or recurrent day
OR EXTRACT(DOW FROM reference_ts) = (SELECT "Notes"::integer FROM "LookUp" WHERE "Id" = wc."DayOfWeek") --corresponding weekday
) AND wc."CalendarDef" = calendar_def AND wc."Status" = 'A'
ORDER BY priority."Code"::integer ASC
LIMIT 1
INTO time_slots;