I have already googled the message, and I have plenty of disk space available on the SVN server (it's about 4% usage of 150 GB).
I have noticed that when I try echo $TMPDIR at the command prompt on the SVN server I get nothing.
What is making this a little confusing is that I only get this message from one location when I do an svn diff (that I've tested so far) - this error is not coming up when I try from three other computers (one of which is testing against the exact same repository, the other two are different repositories on the same svn server).
About the only difference I can see is that the broken working copy is connecting to the server by an IP address where all the others are using a server name (although this resolves over DNS to the same IP Address).
I'm hoping that I don't have to scratch the broken working copy and checkout a new one - unfortunately, this is a legacy project and not all changes have been properly revisioned.
You also get this error if the disk is full, I discovered.
Free up some space and the error disappears.
Eventually figured a method to do this (as well as getting everything currently unrevisioned under proper version control):
I reckon there must have been some corruption in one of the .svn files for that working copy.
On the computer with the problem, try deleting manually that directory, do not commit, but update your svn in your local machine, redo the changes on that directory, it is one directory. That usually works with me on similar errors.
I was able to fix this problem by running the script provided here:
https://svn.apache.org/repos/asf/subversion/trunk/contrib/client-side/svn-clean