I have this on the backs of my arms and my butt
It makes sense to me it's a vitamin deficiency. I found out recently (about 4 months back) that I'm gluten intolerant and cut gluten and my arms and butt are now much smoother than they've been in years. From what I've read, people with gluten intolerance have trouble getting all the nutrients they need because the gluten is messing up their digestion.
Also gluten intolerance is common in Europe (Ireland in particular has a lot of diagnosed celiacs - not sure of the numbers in Germany) so the Celtic connection would also make sense.
I also used to get eczema on my hands. I thought it was connected to dairy because when I ate dairy I'd get it. I cut gluten and after a few months, I can now eat dairy with no problem and no eczema.
Basically all *my* skin problems seem to be gluten, but that's me.
Oh, and my 5 year old son's cradle cap has started to FINALLY go away after I reduced his gluten intake (not really on purpose but since I can't eat it I don't feel like having it in the house so the kids are being cut back). I think that's related since it's also skin.
Edited to say that pregnancy (along with other stressors) is often cited as a trigger for gluten intolerance or celiacs. A person can either have the tendency lying dormant or they are sort of low level reacting but it gets worse after pregnancy. That was definitely true for me. I'd had digestive problems and skin problems for years but it wasn't til after my second pregnancy that things seemed to go a bit haywire.