Modern electronics can do some pretty weird things when battery voltage gets too low, or outside of tolerances.

Now it's working, measure the voltage across the battery terminals and make a note of it. If (let's hope not) it happens again (the immobiliser problem), measure the battery voltage again. If there's a difference between the two, it'll help to confirm the problem is with a voltage drop. Then you need to ask yourself where/why the voltage is disappearing!

Maybe worth checking to see if anything is leaking voltage to earth? Is the bike charging at the correct voltage? Perhaps it's time for a new battery?

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