Array to string conversion by switching PHP Version

I switched to PHP 8.0. On other projects it worked flawlessly. Everything in the admin area seems to work, except for the content management (pages). Temporarily I switched to PHP 7.4 to work with GRAV. However, in this project I get the following error:

An exception has been thrown during the rendering of a template ("Array to string conversion").

/user/plugins/form/templates/forms/fields/select/select.html.twig

value="{{ akey }}"
>
{{ avalue|raw }}

{% elseif item_value is iterable %}
{% set optgroup_label = item_value|keys|first %}

{% for subkey, suboption in field.options[key][optgroup_label] %}
{% set subkey = subkey|string %}
{% set item_value = (field.selectize and field.multiple ? suboption : subkey)|string %}
{% set selected = (field.selectize ? suboption : subkey)|string %}

{{ suboption|t|raw }}

{% endfor %}

{% else %}
{% set val = (field.selectize and field.multiple ? item_value : key)|string %}
{% set selected = (field.selectize ? item_value : key)|string %}
{{ item_value|t|raw }}
{% endif %}
{% endfor %}



{% endblock %}

Arguments

"An exception has been thrown during the rendering of a template ("Array to string conversion")."