Airport 6.3.1 - Unexpected error connecting to Airport Express
I have an Airport Extreme (A1408) base station. I am trying to add an Airport Express (A1392) to my network using Airport Utility 6.3.1 (631.4) with Mac OS/X 10.8.4 (12E 55) running on an iMac. The Airport Utility can find the Express, but returns "An unexpected error occurred. Try again." I have reset the Express using the pinhole without success. I also tried connecting another Airport Extreme (A1408) to the same network with the same result: it can find the device, but returns an unexpected error.
Based on others' posts, I have tried changing the Network setting for IPv6 to "Link-local only". This also did not resolve the problem. Any suggestions are welcome.
iMac, Mac OS X (10.5.5)