There seems to be a lot of rumours and speculation but little else, there are some important questions that has to be asked:
Why was the decision made to acid wash, this is usually done to remove severe build up of scale, so what was found beforehand? We can deduce from this that there was a lot of scale. If this was the case the Water treatment being used was almost totally ineffective.
If it was full of scale once it was removed by the acid wash this could quite likely reveal significant corrosion not detectable beforehand. Once again if corrosion was found after removal of scale deposits the Water Treatment was totally ineffective.
Plenty of locomotive Boilers have been acid washed in the past with no ill effects, some that readily come to mind are SRV's D3 658, 3801 (when first moved to the RTM) and PRR's SMC1, so it is not fatal to a loco Boiler to use this process.
Not having seen it personally I can only speculate like others, but I would suspect there was significant scale deposit bad enough to warrant an acid wash (maybe asked for by the Boiler Inspector) and once this was removed it revealed significant corrosion. Scale deposits may also have been blocking up the leaks mentioned in one post earlier.