Unable to add an particular external reference to a particular project - very strange bug

Hi all,

I've come across a very strange bug.

I have a version controlled directory A, which includes a subdirectory B
to which I'd like to add an external reference to project C. When I try
to update A or B I get this message.

---
Fetching external item into 'ImaServiceManager'
svn: Working copy 'ImaServiceManager' locked
svn: run 'svn cleanup' to remove locks (type 'svn help cleanup' for
details)
---
 
Now, if I check out B by itself it works!  If I check out A (and
therefore B) then it doesn't, even if I then update B by itself.  I
don't understand how the parent directory A can have this influence on
B.  Note that A only contains about 10 files.
 
I've tried everything I can think of, I've cleaned up, deleted and
recreated the directories on the disk (not the server), etc. 
 
The odd thing is that I *can* add that same external reference C to
another similar directory structure and it works perfectly well.  I've
copied the external reference from one property into the other, so I'm
100% sure that the paths are the same.
 
I can also *successfully* add another external references to directory
B, and they work too.
 
In other words the problem is limited to one particular combination of
directory structure and external reference.
 
I'm using version 1.5.2 of Subversion, on Windows XP.
 
Anything that you can suggest to help me would be greatly appreciated.
For obvious reasons I can't can you recreate the bug since it's working
in all other cases.
 
Tim
 
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