There was at least one firmware update (currently at 3.53) and some fixes in the release notes are similar to what I described in my previous post.
I've updated to that versionen but didn't change my setup in the last ~ 6months, so I can't tell if the problems were solved or not.
They use dual port sas drives and two expandeers? :D
A sas switch are cascaded expanders with sas zoning enabled/configured
Microsoft supported this storage spaces setup with dual ported sas devices and dual expanders