[imp] special characters schon in & format - Warning: preg_replace()

[Andreas Moroder]

Hello,

from time to time I get mails imp does not display correctly.
Instead of the character the HTML code is displayed e.g. &egrave or 
&igrave and so on.

At the top of the windows this text is displayed:

Warning: preg_replace(): Unknown modifier '\' in 
/srv/www/htdocs/horde/lib/Horde/MIME/Viewer/richtext.php on line 135


The text is not part of the body but part of a attachmet with this header
----boundary_104650_a098a2de-6573-4610-bd0f-2621e14244c3
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: base64

I use Firefox and I have set it to UTF-8.

Can anyone tell me why the characters are not displayed the way they 
should ?

Additional information:
My mail client thunderbird shows complete garbage.

If necessary I can forward the mail

Thanks
Andreas