Open Directory on Leopard server not recognized after client upgrades to Lion

Hello,

This question originates with the same symptoms as https://discussions.apple.com/thread/3270113 [Can you bind a Lion (1.7.1) Client to a Leopard (10.5.8) Server].

However, I didn't find anything SSL-related to be enabled, so I don't think that the solution applies. (SSL is unchecked in Server Admin, and I have to admit that I didn't find an application called Server.app, since I don't think that it's in my Leopard server installation.)

So the question remains.

We recently upgraded some iMacs and 2009 Macbooks to Lion 10.7.4. However, since the upgrade, these Lion machines don't see/recognize the Open Directory Service on the server anymore... I get a

connection failed to the directory server. (2100)

The other clients (10.6.8) still function perfectly, and logging in via network accounts is no problem.

The server is an xserve on 10.5.8, all machines are up-to-date.

I am really running out of ideas here... has anyone had similar issues? Any help would be much appreciated!
Thanks!