Two things to try:
- During the extended network set up phase temporarily connect each of the extending base stations to the base station that they will be extending.
- Perform a "factory default" reset on each of the extending base stations.
I have posted this elsewhere:
I am having the same problem with Mavericks, repeatedly getting the "An unexpected error occurred. Please try later" or something similar. And I kept trying again and again and again and again with the same result.
So I booted up my iMac (Processor 2.7 GHz Intel Core i5) with my old Mountain Lion disk and, using the Airport Utility 6.2, I was able to set up the Airport Express on the first try. No unexpected errors.
From a troubleshooting standpoint, I have to posit that something is awry either with Mavericks itself or the new Airport Utility 6.3.2. regarding the setup of the Airport Express. The Airport Extreme doesn't seem to encounter the same problems, but then again I have made only a few minor changes. Nothing involving setup. (And I'm not going to)
If you can boot up under an older OSX, I'd suggest you try that first, and then make sure your Airport utility is in the 6.2 range, not 6.3.
I think this is strictly a software problem, either or both with the Airport Utility or the newest OSX.