I reckon this does it:
SELECT mx.Ticket_Closed_Date, mx.Interval, op.*
FROM ( -- create a matrix containing all intervals from Tijdsintervallen and all dates from Tussen_Tickets
SELECT *
FROM Tijdsintervallen
CROSS JOIN (
SELECT DISTINCT Ticket_Closed_Date = CAST(Ticket_Closed_DateTime AS DATE)
FROM Tussen_Tickets
) d
) mx
OUTER APPLY (
SELECT tt.Event_Channel,
DVBIntern = SUM(tt.DVBIntern),
DVBExtern = SUM(tt.DVBExtern),
Tijdsduur = SUM(tt.Tijdsduur),
WrapUp = SUM(tt.WrapUp),
OnHold = SUM(tt.OnHold),
Talk = SUM(tt.OnHold),
RowsAggregated = COUNT(*) -- not in spec but handy to have
FROM Tussen_Tickets tt
CROSS APPLY (
SELECT
Ticket_Closed_Time = CAST(Ticket_Closed_DateTime AS TIME(3)),
Ticket_Closed_Date = CAST(Ticket_Closed_DateTime AS DATE)
) ta
WHERE ta.Ticket_Closed_Time BETWEEN mx.beginTijdsinterval AND mx.eindTijdsinterval
AND Ticket_Closed_Date = mx.Ticket_Closed_Date
GROUP BY ta.Ticket_Closed_Date, tt.Event_Channel
) op
ORDER BY mx.Ticket_Closed_Date, mx.Interval, op.Event_Channel
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden