The Build application uses .svn\entries file to obtain revision information directly from the SVN "entries" file. However, the SVN working copy metadata format recently changed and the build project is now broken.
I worked around the problem by changing the Revision() function as follows:
static string Revision()
using (var reader = new IO.StreamReader("../../../../.svn/entries"))
Note that I an not sure what the first line of the entry files contains, just that it returns "12" for me (which allows the build to proceed). Perhaps this dependency can somehow be removed to prevent issues like this in the future?