Interesting, I have just developed the same problem with my S4.
It is about 4 years old now, was working perfectly.Never before have I used a winter battery conditioner, just let it kill the battery and buy a new one in summer. Then this winter came and I decided to fit a trickle charger for the winter hybernation.
Guess what, now it wont disarm. Phoned Datatool and they told me to throw it away and but a new one. Sent an email to their Support Center with regards to sending the unit and fob back to them for recoding as it is printed in the user book.
Guess what " To this day I am still waiting for Datatool to respond"
Saddly it would seem the only recourse is to take the cursed thing out,
at least that way I can get to ride my bike.
Very simple to remove, simply open the harness up and trace the wires back using a bike wiring diagram. Simmmplessss