Mautic ignores LP template

Your software
My Mautic version is: 5.0.3
My PHP version is: 8.0 or 8.1 (issue is the same on both)
My Database type and version is: MariaDB 10.6.17
My webserver: LiteSpeed Webserver 6.2.1 on Plesk Obsidian 18.0.58

Your problem
My initial issue was: With my custom template, that is already mentioned in this topic:

The template isn’t used by the builder, I mistakenly thought it is solved with the proposed solution, but the issue keeps occuring.
Instead of my template the Builder uses the “Blank”-Template.

This is my code:
config.json

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

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>
		{{ outputScripts('bodyOpen') }}
		{% block content %}{% endblock %}
		{{ outputScripts('bodyClose') }}
	</body>
</html>

page.html.twig

{% extends "@themes/"~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 %}

After restarting the web server I ran into this issue:

To me the HTML seems valid. It pretty much is a blank template with fonts added - I can’t find the issue. Does someone see where I made my mistake? Trying to figure this template out for 2 weeks now…

These errors are showing in the log:
The only thing initially showing up in the console is: “setThemeHtml - Request failed: error builder.js:87:14
Console after restarting the web server:

Steps I have tried to fix the problem:
Changes to config.json and page.html.twig, checking structure of Blank theme and replicating the structure in my template.
Also set the default template in the Configuration to my template.
Restarted my web server and the issue changed as described above.
cleared the cache