PanID the same
Ap=2
ZS=2
Baud 9600
JV =1 (tried with 1 and 0 to no avail
Router API (both set as this)
latest firmware versions
Check your SC value and adjust the S2B to match the S2 and you will be fine.
For the available values of SC, refer page number 133 of the following product manual, http://ftp1.digi.com/support/documentation/90000976_S.pdf