It's a fault that affects the earlier mod state AHRSs - they can't process the ADC input and effectively treat the value as 'off scale'. The AHRS feeds the VSI and so in this condition it shows 9999 ft/min RoC.
I'm surprised to hear the fleet was grounded because a) I thought they had the newer AHRS (or at least the latest a/c did) and b) the same thing happened in Norway last December and an operational solution was agreed between the regulator and operators, with SAC input, which I thought the UK operators had adopted too.
It's unrelated to the EC-225 problem which is at the opposite end of the scale - there is a density altitude limit (2000 ft) for take off, which cold and low pressure conditions can produce at low take off elevations.