BadHeaderError using Flask with FlaskMail and Postfix on Ubuntu server

Question

I'm running a fullstack website with Flask and SQLite. On the page I have a contact-section to let visitors send me an email. The contact-section consists of an HTML form which sends the information to the webserver via HTTP POST. The server itself uses the FlaskMail plugin to create the message and send it to the mail-server (in this case a postfix server I set up on the same server). However, I'm getting a BadHeaderError every time I click submit. I already looked for existing fixes and found an existing Stackoverflow question here. However, the solution there seems to be unrelated to my issue.

Relevant code from webserver.py (Flask python file):

app = Flask(__name__)
app.debug = True

app.config['MAIL_USERNAME'] = 'admin'
app.config['MAIL_PASSWORD'] = 'password'
app.config['MAIL_USE_TLS'] = True

mail = Mail(app)

@app.route('/contact', methods=['GET', 'POST'])
def contact():
    lang = get_lang(request.headers)

    if request.method == 'GET':
            if lang == 'de':
                    return render_template('contact.html', lang=lang, title=title_de, descriptio$
            else:
                    return render_template('contact.html', lang=lang, title=title_en, descriptio$
    elif request.method == 'POST':
            name = request.form['name']
            email = request.form['email']
            company = request.form['company']
            message = request.form['message']

            if not name:
                    flash('Please enter your name')
                    return redirect(url_for('contact'))
            if not email:
                    flash('Please enter your e-mail address')
                    return redirect(url_for('contact'))
            if not message:
                    flash('Please enter a message')
                    return redirect(url_for('contact'))

            message += '\n\n'
            message += ('Name: ' + name)
            message += '\n\n'
            message += ('Company: ' + company)

            msg = Message(message, sender=email, recipients=['pawelczyk.johannes@gmail.com'])

            mail.send(msg)

            flash('You will receive an automatic confirmation mail')
            return redirect(url_for('index'))
    else:
            flash('Bad request')
            return redirect(url_for('index'))

Apache error log: pastebin

HTML:

<div id="contact-form-container">
    <form id="contact-form" action="{{ url_for('contact') }}" method="post">
        <div>
            <div class="contact-form-row">
                <h3 class="to-left">Name:</h3>
                <input class="to-right" type="text" name="name" placeholder="Name">
            </div>
            <div class="contact-form-row">
                <h3 class="to-left">E-Mail:</h3>
                <input class="to-right" type="text" name="email" placeholder="E-Mail">
            </div>
            <div class="contact-form-row">
                <h3 class="to-left">Firma (optional):</h3>
                <input class="to-right" type="text" name="company" placeholder="Firma">
            </div>
            <h3 class="gap-over">Ihre Nachricht:</h3>
            <textarea cols="40" rows="10" name="message"></textarea></br>
        </div>
        <div class="middle">
            <input class="button" type="submit" value="Senden">
        </div>
    </form>
</div>