[Exim] special characters in autoreply?

Hello,

some of our customers are getting the following error from the autoreply
tzransport:

   xxx@???
     Expansion of "Re: $header_subject: (autoresponse)" in
     address_autoreply transport contains non-printing character

What are non-printing characters in this case? 8bit characters? Or \x00?
How can I allow such characters? The last posting I found on this was of
1999, I guess a lot changed since then.

Regards
Marten Lehmann