I recall looking at this before on mine as CANBUS was new to me as I'm an old fart Engineer.
Check if the RING SENSOR (R1200 Ring Antenna on google) is operational (known failures) it's the annular ring around the main key switch, use the spare key to see if this is KEY related (keys are induction charged via the ring and also senses the code written into every key (that's the BMW Immobiliser!) - Keys are £40 each from BMW as I had a fault on mine, you need your VIN via a Dealer with your V5 and ID, 3 days from Germany ONLY SOURCE). I'm sure if you search the issue on the interweb, it will give more info.
Each of the 5 conditions needs to be met to start - Key switch on/Side stand up/Kill switch on/Ring sensor detected/Gear position sensor (Neutral) - must all be detected to start it. Each of them can fail, although not to the extent of anything like other failures and can be rare. If you have someone nearby with a GS911, this is the only way you will see these being ON/OFF or you will have to take it to BMW to diagnose. I never got an error code if I recall when my key failed, thankfully I was leaving for work when it failed to start.
CANBUS can be a PITA to diagnose without a method of checking (GS911) what is ON and OFF, they are all now separate circuits than on the older bikes having each part form part of it. If you are electrically bent, this is now all 0's and 5's (Logic/voltage) Normally open and normally closed, rather than 12v found at source, it's all-transistor heaven.
Good hunting and let us know how you got on.
Edit - I keep forgetting the another condition because it is disabled on my ex-Police bike, but the microswitch in the clutch lever...
