I fixed it as follows (Odoo 11 CE)

Activate Developer mode

Under Settings > Mail Template, open template "Notification EMail"

Click Edit

Click the button for Code view

remove all the code and replace with something very simple such as:

<p style="color: #555555;">

Email sent by

</p>

Save

Test by sending the contact a message. It worked, and the mail was sent without errors.

I then went back to the Notification Email template, edited it, selected the code view again, and pasted in the code from the English version of the template.

That code, in case you need it, is as follows for 11CE:

<p>

% set company = ctx.get('company', user.company_id)

</p><div>

% if ctx.get('has_button_access'):

<div itemscope="" itemtype="http://schema.org/EmailMessage">

<div itemprop="potentialAction" itemscope="" itemtype="http://schema.org/ViewAction">

<link itemprop="target" href="${ctx['button_access']['url']}">

<link itemprop="url" href="${ctx['button_access']['url']}">

<meta itemprop="name" content="View ${ctx['model_name']}">

</div>

</div>

% endif

% if not ctx['is_discussion'] or not len(ctx['actions']) == 0 or ctx.get('has_button_access'):

<div summary="o_mail_notification" style="padding: 0px; width:600px;">

<table style="border-spacing:0px;width: 600px; margin-top: 5px;" cellspacing="0" cellpadding="0" border="0">

<tbody><tr>

<td valign="center">

% if ctx.get('has_button_access'):

<a href="${ctx['button_access']['url']}" style="padding: 8px 12px; font-size: 12px; color: #FFFFFF; text-decoration: none !important; font-weight: 400; background-color: #875A7B; border: 0px solid #875A7B; border-radius:3px">${ctx['button_access']['title']}</a>

% endif

% if ctx.get('has_button_follow'):

% if ctx.get('has_button_access'):

|

% endif

<a href="${ctx['button_follow']['url']}" style="color: #875A7B; text-decoration: none !important;">${ctx['button_follow']['title']}</a>

% elif ctx.get('has_button_unfollow'):

% if ctx.get('has_button_access'):

|

% endif

<a href="${ctx['button_unfollow']['url']}" style="color: #875A7B; text-decoration: none !important;">${ctx['button_unfollow']['title']}</a>

% endif

% if ctx.get('actions'):

% for action in ctx['actions']:

|

<a href="${action['url']}" style="color: #875A7B; !important;">${action['title']}</a>

% endfor

% endif

</td>

<td valign="center" align="right">

<img src="/logo.png?company=${company.id}" style="border-style:none;border-image-width:1;border-image-source:none;border-image-slice:100%;border-image-repeat:stretch;border-image-outset:0;border-left-color:currentcolor;border-bottom-color:currentcolor;border-right-color:currentcolor;border-top-color:currentcolor;padding: 0px; margin: 0px; height: auto; max-width: 80px; max-height: 40px;" alt="${company.name}">

</td>

</tr>

<tr>

<td colspan="2" style="text-align:center;">

<hr style="border-style:solid none none none;border-image-width:1;border-image-source:none;border-image-slice:100%;border-image-repeat:stretch;border-image-outset:0;border-left-color:currentcolor;border-bottom-color:currentcolor;border-right-color:currentcolor;background-color:rgb(204,204,204);border:medium none;clear:both;display:block;font-size:0px;min-height:1px;line-height:0; margin:10px 0;" width="100%">

% if ctx.get('subtype') and ctx.get('subtype').internal:

<p style="background-color: #f2dede; padding: 5px; margin-bottom: 16px;">

<strong>Internal communication</strong>: Replying will post an internal note. Followers won't receive any email notification.

</p>

% endif

</td>

</tr>

</tbody></table>

</div>

% endif

<div>

${object.body | safe}

</div>

% if ctx.get('tracking'):

<ul>

% for tracking in ctx['tracking']

<li>${tracking[0]} : ${tracking[1]} -&gt; ${tracking[2]}</li>

% endfor

</ul>

% endif

% if ctx.get('signature'):

${ctx['signature'] | safe}

% endif

<br><p></p><p>

</p><p style="color: #555555;">

Sent by

% if ctx.get('website_url'):

<a href="${ctx['website_url']}" style=" color: #875A7B;">

% endif

${ctx.get('company_name')}

% if ctx.get('website_url'):

</a>

% endif

using

<a target="_blank" href="https://www.odoo.com" style=" color: #875A7B;">Odoo</a>.

</p>

</div><p></p>

Hi, it's the one called "Notification Email" with field "Applies to" = Message.