I have this problem exactly, so I will be interested to read the answers to your question.

Ojib, may be we will learn, that this is not a problem at all, but normal colour of leaves in this variety.:)

Iron/manganese deficiency, ladies - this plant likes acidic soil conditions - water on Sequestrene Iron Tonic (not sure you can get that, Katarina, where you are) at the roots. May need to do it at least once a year, if not twice.

Thank you Bamboo, I knew you would know :)) Yes, I do not have that product here, but I added today 10 litres of peat and some organic fertiliser for conifers. Do you think it can help?

It might help a bit, but you won't get such good results without something like Sequestrene - maybe they do a similar product there but called something completely different?

Hi Bamboo, unfortunately I did not find any translation for what does it mean Sequestrene. Or is it trade mark?
Anyway, what does it contain?

Luckily, I have Sequestrene in the shed. Thanks, Bamboo.
I put it on a yellowing Skimmia once before and could not believe the difference it made.

Contains chelated iron, available either as a mix to apply by watering can, or granules to be added to the ground around the plant. Google Sequestrene for more info - although there isn't much more than what I've said.

I have found we have something like that but for application on leaves.

Well, that might work perfectly well, I'd give it a try... but I imagine as its a foliar application, it may need repeating more often.

Copperas (iron sulphate) and manganese sulphate could be helpful, if the soil pH isn't 8.0 or higher. Usually longer lasting than Sequestrene, too. Eventually, the peat will also work, though you will need to keep applying it, or a somewhat acidic compost twice a year for several years for the full effect. Compost tea--as Rogi uses--will work more quickly, but needs more frequent application.