I think someone may have already mentioned this, but have you looked into thoracic outlet syndrome? Being in Germany, I don't know if you would have access to a vascular surgeon who is knowledgeable about TOS to do a proper work-up. Regular neurologists or ortho docs don't know what to look for. You seem to have a combination of sensory and motor issues (much like me!) that do not have an easy explanation. It is horribly frustrating, I know. It is entirely possible that you won't find out one "true" cause of what is happening, and you may join the "idiopathic" club.

For me, it's gotten to the point that I was driving myself crazy trying to find answers, but all the dr.'s and tests weren't giving any. I'm actually laying off the search for awhile. Trying to eat a really healthy diet, doing gentle exercise and PT and willing my body to heal itself. Sounds New-Agey, but I'm trying to save my sanity!