Hermann, alias Lucky Luke !

I confirm it's quite rare in France too.

For example, I didn't find it in my area, while I was looking for it.

Ya, I think there are two factors: first, they need old living Fraxinus trees. and experience from Scandinavia (Norway, Sweden) where it is much more common than in Central Europe show that it is especially found on the bark of old, pollarded living Fraxinus trunks that are situated at forest margins, or on solitary trees in meadows, parks, etc. Second, it seems to be more common in areas with mild climate and, at least seasonally, high humidity. And as especially old Fraxinus trees are severely declining and disappearing in most of Europe due to ash dieback decline, it is expected to become much rarer and even endangered over much of its current distribution range...