Random images from Media-DB get lost after packing the Media-DB

Hi!

We're using ZMS and our image files are stored in a Media-DB.
Generally, that's fine.
But when we pack our Media-DB (we do this about once a week), some
random images seem to get lost.

We have error messages like this in our logs:

OSError: [Errno 2] No such file or directory:
'/var/lib/zope/instance/usb/var/media/usb//zv_118762757295.gif'

However, a file named 'media/usb/zv_118762757296.gif' does exist.
The IDs seem to get async for an unknown reason. Also, the
double-slash looks a bit strange.

Does anybody else have similar problems? Do you have any ideas how we
can avoid these problems?

Unfortunately, the only "solution" we have at this time is to upload
the images again (and again, and again...).
But a real solution would be highly appreciated.

Our system configuration (running Debian stable):
ZMS: ZMS 2.9.2-a29 (Build #124b)
ZOPE: (Zope 2.9.6-final, python 2.4.4, linux2)
PYTHON: 2.4.4 (#2, Apr 5 2007, 20:11:18) [GCC 4.1.2 20061115
(prerelease) (Debian 4.1.1-21)]

Thank you for your help!