[exim] Error failed to expand

Hi list!

I want to update my Exim (previous version 4.75, now 4.88).
I compiled the new version on a Debian Jessie Server.

Now if I try to use my old configure-file, I get the error:

cannot be resolved at this time: localWrite_gw router failed to expand
remove_headers item "NULL": ":" expected after "def"

the line with the error is that:

headers_remove = ${if def:h_X-DefSubject {Subject}}:X-DefSubject:

which worked on Exim 4.75.
I can't see any error. Can someone help me?

Thanks a lot
Luca Bertoncello
(lucabert@???)