Why is svnserve not linked against libldap-2.4-2 any more?

I tried to migrate from 11.04 to 12.04 but svnserve does not validate users against sasl's ldap auxprop plugin any more.

in /var/log/auth.log:
 svnserve: could not find auxprop plugin, was searching for 'ldap'

... whenever I open a Tortoise-SVN repo-browser on a client machine with a URL pointing to this server.
Which does work with the 11.04 server.

But by using testsaslauthd I can auth against ldap without a problem, so I have no reason to question saslauthd.
Also, The error says "svnserve: could not...", not "sasl...".

Due to the identical configuration of my 11.04 and 12.04 installations, the only possible reason I could find is the missing linking:

libldap_r-2.4.so.2 => /usr/lib/libldap_r-2.4.so.2

... which is present under 11.04 (1.6.12dfsg-4ubuntu2.1) but not under 12.04 (1.6.17dfsg-3ubuntu3).
I DO have installed the libldap-2.4-2 on both systems.

Although i have to admit, there are more libraries that are linked under 11.04 but not 12.04:

libldap_r-2.4.so.2 => /usr/lib/libldap_r-2.4.so.2 (0x00007f22fb596000)
liblber-2.4.so.2 => /usr/lib/liblber-2.4.so.2 (0x00007f22fb387000)
libresolv.so.2 => /lib/x86_64-linux-gnu/libresolv.so.2 (0x00007f22f9ed7000)
libgssapi_krb5.so.2 => /usr/lib/x86_64-linux-gnu/libgssapi_krb5.so.2 (0x00007f22f9ca2000)
libgnutls.so.26 => /usr/lib/x86_64-linux-gnu/libgnutls.so.26 (0x00007f22f9a00000)
libgcrypt.so.11 => /lib/x86_64-linux-gnu/libgcrypt.so.11 (0x00007f22f9786000)
libkrb5.so.3 => /usr/lib/x86_64-linux-gnu/libkrb5.so.3 (0x00007f22f94c1000)
libk5crypto.so.3 => /usr/lib/x86_64-linux-gnu/libk5crypto.so.3 (0x00007f22f929a000)
libcom_err.so.2 => /lib/x86_64-linux-gnu/libcom_err.so.2 (0x00007f22f9096000)
libkrb5support.so.0 => /usr/lib/x86_64-linux-gnu/libkrb5support.so.0 (0x00007f22f8e8d000)
libkeyutils.so.1 => /lib/x86_64-linux-gnu/libkeyutils.so.1 (0x00007f22f8c8a000)
libtasn1.so.3 => /usr/lib/x86_64-linux-gnu/libtasn1.so.3 (0x00007f22f8a79000)
libgpg-error.so.0 => /lib/x86_64-linux-gnu/libgpg-error.so.0 (0x00007f22f8874000)
librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f8f32ea6000)

I also Do have installed all those library packages on both systems.
And subversion-tools.
I also re-installed subversion.

So why is it not linked? Is this a bug? If not, why is it not authenticating?

thanks,
klaus