[horde] can not get attachement (got 0Ko file)

[chmick]

hello 
I have a weird problem with horde and imp. 
I installed horde on my linux server (debian etch) running on apache2 (https)
with php5 and imap
I can see may mail, send mail with attachement (up to 2 Mg).
BUT (there is always a but) If I try to see an attachement or to download an
attachment I got a blank page or a 0Ko file. I don't see anything wrong in the
log  from horde3 or apache2. I probably forgot something but I don't know what
and I'm running out of ideas. 
Can anyone give me a clue or an hint about this problem ??

Thanks

package used :
horde3   3.1.4-1 
imp4     4.1.4-1
turba2  2.1.4-1