Timezone in email formatted wrong

The output for a date time produces an ascii code in a text email when you use

Started: {{ .StartsAt }}

Expected Output:
Started: Fri, 26 Apr 2024 17:15:40 +0200

Real output in e. g. Outlook
image

Only switching away from numeric offset to the timezone name gets a proper result
by using e. g.

Started: {{ .StartsAt | date “Mon, 02 Jan 2006 15:04:05 MST” }}

Am I doing something wrong?