i found deinstalling FM10, then reinstalling same copy from same installer cd on osx10.5.8 (on xserve) to be impossible.
What I did:
-run deinstaller - OK
-manually deleted /Library/Filemaker Server/
- permission repair
- emptied java cache
- run installer - results in "installation succesful. Do you want to deploy FM now?"
Clicking yes, returns in safari not able to connect to localhost:16000
Problem: unable to connect to serverd
executing: fmserver_helperd results in:  CreatePIDFile: couldn't open file for writing: /Library/FileMaker Server/Database Server/.fmserver_helperd.pid omniORB: Error: Unable to create an endpoint of this description: giop:tcp::50006 2010-04-21 12:09:41.665 +0200 [HelperApp] AppBase::GetRootPOAObjects() gets a CORBA::INITIALIZE exception. omniORB: ERROR -- the application attempted to invoke an operation on a nil pseudo-object reference. terminate called after throwing an instance of 'omni_thread_fatal'