Cannot login to computer after upgrade to Lenny

[MoS <mosdev@free.fr>]

Hi,
I finally upgraded my computer to Lenny, following the release notes advices, and everything seemed to have worked quite well.
But after the reboot, I cannot login anymore !
The system complains about a missing library (libdl.so.2) ??

A "failsafe" reboot using new Lenny kernel (2.6.26) provides me a root console, but I cannot do much more here (see the log below), I get the same error using "su".

I have no clue about what's happening, and I couldn't find anything on the internet about this problem...
Some more details :

<sources.list>
deb http://ftp.fr.debian.org/debian/ lenny main contrib non-free
deb-src http://ftp.fr.debian.org/debian/ lenny main contrib non-free

deb http://security.debian.org/ lenny/updates main contrib non-free
deb-src http://security.debian.org/ lenny/updates main contrib non-free

# Backports
deb http://www.backports.org/debian lenny-backports main contrib non-free

<log>
# su
ksh: error while loading shared libraries: libdl.so.2: cannot open shared object file: No such file or directory

# ldd /bin/su
        linux-gate.so.1 =>  (0xb7fca000)
        libpam.so.0 => /lib/libpam.so.0 (0xb7fbd000)
        libpam_misc.so.0 => /lib/libpam_misc.so.0 (0xb7fba000)
        libc.so.6 => /lib/i686/cmov/libc.so.6 (0xb7e5f000)
        libdl.so.2 => /lib/i686/cmov/libdl.so.2 (0xb7e5b000)
        /lib/ld-linux.so.2 (0xb7fcb000)

# su - www-data
-su: error while loading shared libraries: libdl.so.2: cannot open shared object file: No such file or directory
# ls -la /lib/libdl.so.2
0 lrwxrwxrwx 1 root root 12 May 31 13:09 /lib/libdl.so.2 -> libdl-2.7.so

# ls -la /lib/libdl-2.7.so
12K -rw-r--r-- 1 root root 9.5K Jan  4 19:11 /lib/libdl-2.7.so

# dpkg -S /lib/libdl.so.2
libc6: /lib/libdl.so.2

# dpkg -l libc6
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Cfg-files/Unpacked/Failed-cfg/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err: uppercase=bad)
||/ Name           Version        Description
+++-==============-==============-============================================
ii  libc6          2.7-18         GNU C Library: Shared libraries



Has somebody ever met this problem ? Anybody has an idea ?
I beg for helppp !!

Thanks in advance
MoS