Mautic 5: page.html.twig isn't used

Your software
My Mautic version is: 5.0.2
My PHP version is: 8.1.27
My Database type and version is: 10.5.11-MariaDB-1:10.5.11+maria~focal

Your problem
My problem is:
I copied a working theme, modified the .json and .twig files - uploaded it, but it still doesn’t use the page.html.twig file as template for the landing pages.
Instead it uses the page.html.twig of the Blank theme.
It works for the emails though!

These errors are showing in the log:
there are none shown

Steps I have tried to fix the problem:

  1. clearing cache and tmp via SSH
  2. step 1 + restarting the webserver application (LSWS)
  3. removing the theme, step 1 & 2, uploading theme again

page.html.twig:

{% extends ":"~template~":base.html.twig" %}

{% block content %}
	{# HEADER #}
	<div data-section-wrapper="1">
		<center>
			<table data-section="1" style="margin: 16px auto;border-collapse: collapse !important;max-width: 800px;" cellpadding="0" cellspacing="0">
				<tr>
					<td data-slot-container="1">
						<div data-slot="image">
							<img src="{{ getAssetUrl('themes/'~template~'/img/logo.png', null, null, true) }}" alt="Convernatics Logo" width="100%">
						</div>
					</td>
				</tr>
			</table>
		</center>
	</div>

	{# MAIN CONTENT #}
	<div data-section-wrapper="1" style="padding: 10px">
		<center>
			<table data-section="1" style="margin: 0 auto;border-collapse: collapse !important;max-width: 800px;" cellpadding="0" cellspacing="0">
				<tr>
					<td data-slot-container="1">
						<div data-slot="text">
							<h2>Servus {leadfield=firstname},</h2>

							<p>Absatz mit Text</p>
						</div>
					</td>
				</tr>

				<tr>
					<td data-slot-container="1">
						<div data-slot="text">
							<h2>H2 Überschrift</h2>

							<img src="https://placehold.co/600x400" width="600" height="400" alt="Video 1" style="max-width: 100%;">
						</div>
					</td>
				</tr>
			</table>
		</center>
	</div>

	{# FOOTER #}
	{# <footer data-section-wrapper="1" style="background-color: #1fc1b1">
				<center>
					<table data-section="1" style="margin: 0 auto;border-collapse: collapse !important; width: 100%" cellpadding="0" cellspacing="0">
						<tr style="vertical-align: top">
							<td>
								<h4>Ressourcen</h4>
								<ul>
									<li><a href="https://convernatics.com/blog">Blog</a></li>
									<li><a href="https://outlook.office365.com/owa/calendar/Convernatics@peter-zipper.at/bookings/s/ceFuU0IRI0O4uVp2PmCxfQ2" target="_blank">15 Minuten Austausch</a></li>
								</ul>
							</td>
							<td>
								<h4>Company</h4>
								<ul>
									<li><a href="https://convernatics.com/kontakt/">Kontakt</a></li>
								</ul>
							</td>
							<td>
								<h4>Stay in touch</h4>
								<ul>
									<li><a href="https://convernatics.com/ueber-convernatics/" target="_blank" rel="noopener">Über uns</a></li>
									<li><a href="https://www.linkedin.com/in/peter-michael-zipper/" target="_blank" rel="noopener">Peter auf LinkedIn</a></li>
									<li><a href="mailto:einfach@convernatics.com">Schick uns eine Mail</a></li>
								</ul>
							</td>
							<td>
								<h4>Legal</h4>
								<ul>
									<li><a href="https://convernatics.com/impressum/">Impressum</a></li>
									<li><a href="https://convernatics.com/datenschutzerklaerung/">Datenschutz</a></li>
									<li><a href="https://convernatics.com/agb/">AGB</a></li>
								</ul>
							</td>
						</tr>
						<tr>
							<td colspan="4">
								<div style="text-align: center; border-top: 1px solid #df1628; margin-top: 64px">
									<p style="color: #fff; opacity: 0.65">
										© 2024 - Convernatics | all rights reserved
									</p>
								</center>
							</td>
						</tr>
					</table>
				</center>
			</footer> #}
	{dwc=footer}
{% endblock %}

base.html.twig:

<!DOCTYPE html>
<html>
	<head>
		<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">

		<link rel="icon" href="{{getAssetUrl('themes/'~template~'/img/favicon.png')}}" type="image/png"/>

		{% if page is defined %}

			<title>{pagetitle}</title>
			<meta name="description" content="{pagemetadescription}">
		{% endif %}

		{% block stylesheets %}{% endblock %}
		{{ outputHeadDeclarations() }}
		<style type="text/css">

			@font-face{
				font-family: 'Utily';
				src: url(https://l.convernatics.com/fonts/Utily-Bold.woff);
				font-weight:700;
			}
			@font-face{
				font-family: 'Utily';
				src: url(https://l.convernatics.com/fonts/Utily-Regular.woff);
				font-weight:400;
			}

			@media(max-width: 768px) {
				/* Adjust the layout for smaller screens */
				table[data-section="1"] {
					display: block;
					width: 100%;
				}
				td {
					display: block;
					width: 100%;
					margin-bottom: 20px;
				}
				td:last-child {
					margin-bottom: 0;
				}
			}
		</style>
		<link rel="stylesheet" href="https://l.convernatics.com/themes/convernatics/css/style.css" type="text/css">

	</head>
	<body>
		<svg class="antiselect triangle animate__animated animate__slower animate__pulse" viewbox="-10 -10 120 120">
			<path class="animate__animated animate__fadeInLeft" d="M 27 0 L 73 0 C 86.5 0 93.25 13.5 86.5 27 L 63.5 73 C 56.75 86.5 43.25 86.5 36.5 73 L 13.5 27 C 6.75 13.5 13.5 0 27 0 Z "></path>
		</svg>
		<svg class="antiselect square animate__animated animate__slower animate__pulse" viewbox="0 0 100 100">
			<circle class="animate__animated animate__fadeInRight" cx="50" cy="50" r="50"></circle>
		</svg>
		{{ outputScripts('bodyOpen') }}
		{% block content %}{% endblock %}
		{{ outputScripts('bodyClose') }}
	</body>
</html>

config.json:

{
  "name": "Convernatics",
  "author": "Peter M. Zipper",
  "authorUrl": "https://convernatics.com",
  "builder": ["legacy", "grapesjsbuilder"],
  "features": ["page", "email", "form"]
}