Modular page with complex collection exception


I have a weird problem with modular pages when using complex collections. They throw a consistent exception when trying to view them in the admin panel:

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

The error happens at line 69 in user/plugins/admin/themes/grav/templates/forms/field.html.twig

{% block prepend %}{% endblock prepend %}
    {# required attribute structures #}
    name="{{ field_name }}"
    value="{{ value|join(', ') }}"        <- This line
    {% if field.key %}
        data-key-observe="{{ (scope ~ field_name)|fieldName }}"
    {% endif %}

Oddly enough, this page works just fine when viewed on the website, like a regular visitor would see it.
If it helps - I’m using a custom, rather simple theme - based on template created with grav tool, and it’s mostly just customized CSS. I haven’t changed anything manually in system files.

My modular page definition (

title: Some Title
        - '@taxonomy':
            category: js
            tag: player_playlist
        - '@taxonomy':
            category: music
            tag: player
body_classes: modular
twittercardoptions: summary
articleenabled: false
musiceventenabled: false
orgaenabled: false
    ratingValue: 2.5
orgaratingenabled: false
eventenabled: false
personenabled: false
restaurantenabled: false
    priceRange: $

I also tried options like referring to modular pages under several normal pages, like the following code - and ended up with exact same error. The only time the page is displayed in admin panel is with simple one-line option like ‘@self.modular’ or ‘@page.modular’.

    - '@page.modular': '/playlist'
    - '@page.modular': '/player'

I double-checked that taxonomy info is defined in all the pages I need. Assets for “player-playlist-js” are also defined and verified to load. Example from page with playlist scripts:

        - js
        - player_playlist

{% do assets.addJs('player-playlist-js') %}
{{ assets.js() }}

“player” page contains a lot of HTML so I’ll leave it out. The taxonomies are also defined there like in this page.

I tried looking up this error in a similar situation without any results. Hope someone will have some idea about what’s going on here.