Hi Julian,
I am trying to solve the older threads here, at least trying to see if it was solved. If not, please let us know.
For your problem I would first query the server with problem to check if the GUID can be found:
dfsradmin rf list /rgname:testrg /attr:rfname,rfguid.
No Instances found is usually a return when the referred folder is not found in the server.
Please give a look in the following article for more hints about Replicated Folders:
http://blogs.technet.com/b/askds/archive/2007/10/05/top-10-common-causes-of-slow-replication-with-dfsr.aspx
Thank you,
F. Schubert
System Administrator
MCP | Microsoft Certified Professional
MCTS 70-640 | Microsoft Certified Technology Specialist: Windows Server 2008 Active Directory, Configuration
MCTS 70-642 | Microsoft Certified Technology Specialist: Windows Server 2008 Network Infrastructure, Configuration