With such an event handler and the associated operation chain :

<handler chainId="mail_created_chain">
  <event>documentCreated</event>
  <filters>
    <doctype>MailMessage</doctype>
  </filters>
</handler>

<chain id="mail_created_chain">
  <operation id="Auth.LoginAs">
    <param type="string" name="name">Administrator</param>
  </operation>
  <operation id="Context.FetchDocument"/>
  <operation id="Audit.Log">
    <param type="string" name="event">TEST</param>
    <param type="string" name="category">Automation</param>
  </operation>
  <operation id="Auth.Logout"/>
</chain>

I m meeting the following error, only for mailMessages having, for instance, a xlsx attachment

Erreur de connexion Error during Html2Text conversion

The preview through pdftohtml is working fine on these docs.

The problem does not occur with a txt, docs, odt or pdf contaning a scan attachment.

I can't see what may be missing in regard of this answer

No ERROR in logs.

Any idea ?