Kolab 2.2.4: Apache crashes when handling attachments > ~5 MB

[nick at netdome.biz]

Dear all,

currently, my users are experiencing a strange Apache crash when posting
(i.e. writing new or forwarding) mail with attachments bigger than about 5
MB.

Logfile looks as following:

apache-access.log: myhost.mytld - - [11/Feb/2011:10:39:25 +0100] "POST
/client/dimp/compose.php HTTP/1.1" 500 26

which also applies to IMP's compose.php.

OpenPKG install, Kolab 2.2.4. Max. attachment size set to 25 MB. This
happened in main.cf and php.ini.

I googled, but didn't find anything helpful. Has anybody experienced a
similar behaviour and could point me to the right place to
start/look/debug?

thank you,
Nick