This is an unusual question, Almyron, for the UK. The small amount of ash that would have covered your plants should not have hurt them by itself, indeed, as you say, it is beneficial. All I can think of is that the ash was carrying a lot of sulphur with it which became very acidic in the rain.

I would have thought if it was volcanic ash that more gardens would have been affected around you and it would have been news? Especially if it happened 2 years running?
Newly planted plants usually die due to being poor plants or planting conditions such as lack of water?
I really dont think that your garden suffered this fate 2 years running unless other gardens near you did?
Can you give us some more information on what and how you planted them?
Is it possible that you applied weed killer sometime before planting?

If the plants died so quickly, and if it was the ash that caused it, it must have been though damage to the leaves as there wouldn't have been time for anything to get down to root level, though as Bulbaholic says this sounds pretty unlikely. I wouldn't worry about the soil, but in the unfotunate event of it happening again it might be a good idea to hose down the plants in the garden as soon as you can. You don't say whether established plants suffered as well as they newly planted ones?

Volcanic ash which isn't sufficiently watered into the soil can form a crust which prevents the penetration of water and air, resulting in damage to some sensitive plants. That can be fixed with a light coating of compost, raked into the ash layer and watered well. More damage is done by the ash coating the leaves and blocking the light, and the pores that the leaves and stems breathe with. That is easily fixed by brooming the ash off the plants, and then washing them with a strong spray of water, both over and under the leaves. Once it has had a chance to settle in, Volcanic ash is a good source of a number of micronutrients, plus potash and, occasionally, phosphate.

Sorry but I still cannot see this killed his new plants 2 years running?

New ash fall 2 years running, Almyron? Directly on your new plants?

Sorry mate, have just read this question and would suggest maybe you have swallowed too much volcanic ash.

don't know what you are on but you need help, if your plants died so quickly then I think you may also be affected. You must have had the biggest concentration of volcanic ash than any one else in the UK.