Answering my own question:
Apparently my SVN URL had the wrong case! A Google search turned up an article (no longer available online) that explained what was going on. My URL was of the form http://svn.foobar.com/foobar but the actual repository was called http://svn.foobar.com/fooBar.
I use TortoiseSVN, so the fix was to use the Relocate command to correct the path to the repository.
Hopefully this will help someone else.