After applying the incremental installer to upgrade my Domino Server (running on Red Hat Linux 9.0) from 6.5.0 to 6.5.1, I noticed an error displayed on the console during startup where the server was complaining about missing a file ( libzlib.so ) from the location /opt/lotus/notes/latest/linux/http.
The solution was to download and install the libcomprex package.
Download from:
http://www.gnupdate.org/components/libcomprex/index.xml
Download the one under binaries labelled: [HTTP] libcomprex-0.3.3-1.i386.rpm
As root, and from a command line, type:
rpm -i libcomprex-0.3.3-1.i386.rpm
Assuming it installs ok, then do this command to copy the needed file to the right place:
cp /usr/lib/comprex/archive/libzlib.so /opt/lotus/notes/latest/linux/
Restart your server and all should be well. 
Hope this helps!
PS: This also solved the occasional problem I had with outgoing mail failing in the SMTP router due to " unknown OS error "