Critical Error on website after converting datetime fields to timestamps

Hi,
I have a nested cloneable group with dates, for which I have created a view.
It was working fine until I activated the "save as timestamps" option and re-saved the related posts that use those fields.

The posts don't show and I get the regular "Critical Error" WP error instead.
I have enabled WP debug and logging but no error code is shown and the logs are empty.
Also tried to recreate the view from scratch but no luck.

When I remove the timestamp option and re-save the posts, it works fine again.
(I prefer using timestamps though because it allows me to use i18n features.)

What could be the reason?

Here's a stripped-down version of the view code:

{% set field = attribute( post, 'termin-modul' ) %}
{% for clone in field %}

{% set field = attribute( clone, 'termin-modul-daten' ) %}
{% for clone in field %}
	 {% set field = attribute( clone, 'termin-modul-datum-startzeit' ) %}
{{ field | date( 'j. F Y' ) }}
{% endfor %}

{% endfor %}