You mean something like this!?

SELECT 
  DATE_FORMAT(ost_ticket.closed, '%Y-%m-%d') AS monat, 
  -- DATE_FORMAT(ost_ticket.closed , '%Y-%m') AS monat,
  COUNT(DISTINCT ost_ticket.number) as anz_tickets, 
  -- COUNT(DISTINCT ost_ticket__cdata.subject) as hilfethema, 
  -- COUNT(DISTINCT ost_user.name) as anz_kunden, 
  -- ost_ticket.ticket_id,
  -- ost_form_entry_values.value as weiterleitungsgrund,
  ost_form_entry_values.value as bearbeitungsstatus 
FROM 
  ost_ticket 
  INNER JOIN ost_thread_entry ON ost_ticket.ticket_id = ost_thread_entry.thread_id 
  INNER JOIN ost_user ON ost_ticket.user_id = ost_user.id 
  INNER JOIN ost_timesheet ON ost_thread_entry.id = ost_timesheet.thread_entry_id 
  INNER JOIN ost_ticket__cdata ON ost_ticket.ticket_id = ost_ticket__cdata.ticket_id 
  -- Weiterleitungsgrund
  INNER JOIN ost_form_entry ON ost_ticket.ticket_id = ost_form_entry.object_id 
  INNER JOIN ost_form_entry_values ON ost_form_entry.id = ost_form_entry_values.entry_id 
  INNER JOIN ost_form_field ON ost_form_entry_values.field_id = ost_form_field.id 
WHERE 
  ost_ticket.closed BETWEEN "2020-03-16 00:00:00" 
  AND "2020-03-17 23:59:59" 
  -- ost_ticket.closed >= (NOW() - INTERVAL 12 MONTH) AND ost_ticket.closed < NOW()
  -- MONTH(ost_timesheet.created)=03 and YEAR(ost_timesheet.created)=2020 
  -- Abteilungs-Switch (ABT1/ABT2)
  AND ost_ticket.number LIKE 'ABT2%' 
  -- Weiterleitungsgrund
  -- AND ost_form_entry_values.field_id = 42
  -- Bearbeitungsstatus
  AND ost_form_entry_values.field_id = 41 
GROUP BY 
  DAY(ost_ticket.closed), 
  bearbeitungsstatus 
  -- YEAR( ost_ticket.closed ) *100 + MONTH( ost_ticket.closed ) ASC
ORDER BY 
  ost_ticket.closed

you have to adjust at least these parts to fit for your id's/numbers....

AND ost_ticket.number LIKE 'ABT2%' 
  -- Weiterleitungsgrund
  -- AND ost_form_entry_values.field_id = 42
  -- Bearbeitungsstatus
  AND ost_form_entry_values.field_id = 41