Thank you for posting.
Error 6 is for 'File is read-only' according to the FileMaker Pro error codes reference:
Do all users have write permission set on the file? Is the file on a read-only drive? One way to test this is simply by logging in as another user and trying to move the files (without entering an Admin password).
FileMaker Server 5.5 hasn't been tested on Snow Leopard so there could be some issues getting it to work. Some workarounds include upgrading to a later version of FileMaker or running a virtual machine with a supported OS on the Snow Leopard server.