Hi Christoph,
Please check in your system whether you have set:
icm/HTTP/file_access_0 = PREFIX=/sap/public/icmandir/,DOCROOT=$(DIR_ICMAN_ROOT),ARCHIVE=$(DIR_EXECUTABLE)/ITS.SAR
This guarantees that the ICM process will extract the ITS.SAR package, distributed inside the kernel.
You can also check in ($DIR_DATA)/icmandir/its whether the files you got 404 HTTP error actually exist.
Regards,
Cris