Adding a sled to a rack of theseus fails with CannotSpareAShare

On london, I had expunged and added back sleds 14, 15, and 16.
I then expunged sled 17.

Things went well, and after the expungement of 17, I attempted to add it back.

root@oxz_switch0:~# omdb nexus sleds list-uninitialized                                                                            
note: Nexus URL not specified.  Will pick one from DNS.                                                                            
note: using DNS server for subnet fd00:1122:3344::/48                                                                              
note: (if this is not right, use --dns-server to specify an alternate DNS server)                                                  
note: using Nexus URL http://[fd00:1122:3344:123::32]:12221                                                                        
RACK_ID                              CUBBY SERIAL      PART        REVISION                                                        
255b17ef-3bc7-466c-aef2-c70ba0d9ddaa 17    BRM44220007 913-0000019 6                                                               
root@oxz_switch0:~# omdb -w nexus sleds add BRM44220007 913-0000019                                                                
note: Nexus URL not specified.  Will pick one from DNS.                                                                            
note: using DNS server for subnet fd00:1122:3344::/48                                                                              
note: (if this is not right, use --dns-server to specify an alternate DNS server)
note: using Nexus URL http://[fd00:1122:3344:123::32]:12221                                                                        
Error: adding sled                                                                                                                 
                                                                                                                                   
Caused by:                                                                                                                         
    0: Communication Error: error sending request for url (http://[fd00:1122:3344:123::32]:12221/sleds/add)
    1: error sending request for url (http://[fd00:1122:3344:123::32]:12221/sleds/add)
    2: operation timed out

Start by reproducing omdb -w nexus sleds add BRM44220007 913-0000019 and tracing the /sleds/add entry point. Compare the reported CannotSpareAShare behavior with the observed request timeout; done means re-adding the expunged sled completes successfully without either failure.