Sorry for the late answer, I'm on vacation right now. I wanted to set up the home server for external access before my vacation, but unfortunately didn't work due to the above problem.
I started the installation in expert mode and used "something else" for disk partitioning because there is more than one disk and, as I said, some LVM partitions with qemu system volumes.
I only repartitioned the first hard disk and specified it for formatting (including swap).
Ubuntu/Mate installation should not try to format swap partitions within LVM (system) volumes, or search within these volumes at all. In my opinion, a big bug!
I've thought about it a bit in the meantime and will try to install the Ubuntu server distribution after my vacation and then install Mate as desktop. This might work better.
Thank you for help!