Changes have been made in Git for this ticket:

Bug #10275: Silence errors for related parts that don't contain any 
related content

  1 files changed, 5 insertions(+), 3 deletions(-)
http://git.horde.org/horde-git/-/commit/8e350f2789653215f4eb8bfd6116ccdf22729a0a

With the latest IMP release the notice still persist:

2011-08-03T09:16:45+03:00 WARN: HORDE [imp] PHP ERROR: Invalid 
argument supplied for foreach() [pid 31347 on line 170 of 
"/var/www/html/horde/imp/lib/Mime/Viewer/Html.php"]
2011-08-03T09:16:45+03:00 WARN: HORDE [imp] PHP ERROR: array_diff(): 
Argument #1 is not an array [pid 31347 on line 246 of 
"/var/www/html/horde/imp/lib/Mime/Viewer/Html.php"]

Changes have been made in Git for this ticket:

Bug #10275: Ignore the base text/html part when calculating 'unused' 
cids parts

  1 files changed, 1 insertions(+), 1 deletions(-)
http://git.horde.org/horde-git/-/commit/6d5235f211ea6ec5761a8e5a18b5778de4942652

Actually this was only a follow-up error, I've been digging so long 
that I forgot what I really wanted to look for. The original problem 
is that this message show a status message that it contains an 
attachment that cannot be displayed inline (why?). I expect this to be 
the text/plain part. But if I click the link to open it, I get the 
text/html part instead, with a broken message and the error message 
from my first comment in the logs.

The attached message causes notices about wrong arguments to foreach 
in IMP_Mime_Viewer_Html, line 170, because the related_cids meta data 
is not set. AFAICS this would only be set in the related mime part 
from the Related mime viewer, but the part is retrieved here directly 
through IMP_Contents.