Same Problem...
Connecting via Remote Admin is OK - I can see information. Under OS X (10.2.8) and FMP 6 (6.0v4) the Remote Admin window reports that the plug-in, Server Administration X (5.5v1), is the incorrect version, when connected to my Server 5 (5.0v2).
I found the extension 'Server Administration X' (5.5v1) on a Server 5.5 install CD.
--dm