My spare WIU did this same thing to me the other day. I tried it with multiple TIU's, Changed between MTH & HOME modes, and used different serial cables. I finally reloaded the 6.1 Firmware and made sure to not check mark the "Keep Settings" option.
This may or may not solve your problem but it is worth a try.
EDIT & UPDATE: As Engineer-Joe points out below, I meant to say I reloaded the WIU Firmware version 1.1 (not 6.1 which relates to the TIU). I did not have to make any changes or firmware updates to the TIU.