Maybe the aircraft was downgraded to CAT I. Sometimes we have to do practise CAT II/III approaches due to some problem with the AFCS, and then an avionics engineer has to sign it off when enough approaches have been completed. We do not have avionics engineers at every base, but STN definitely would.
My theory is the aircraft was being flown to STN to get signed off by an engineer, STN was CAT II/II when they got there so they diverted as they could not perform an autoland. They are now returning empty to base and the aircraft will remain CAT I only. To keep it simple for the pax, they called it a "software update", which is not too far from reality when you think about it.
I could be completely wrong but it's the only good reason I can think off.