Hi!
Eventually I could solve the problem (using another Linux System):
For reasons completely unknown to me the partitions I had created
manually were not retained (I had a small second partition, and I had
erased both partitions created a second larger partition and two more):
However after successful installation the old partitions were back AND
LVM complained that the physical volume inside the LUKS is larger than
the partition. That was the clue I needed! So I checked the partitions
and found the partition was not what I had created (but the PV was the
one I had created). I could suspect anaconda reading the partition table
first, and then writing it back for no good reason (I said _I_ will
provide partitions).
Well the fix was (time consuming): I created the partitions from the
"other" Linux, then booted the installer again and did yet another
install. Now it booted! From SSD. Now really fast 
So Qubes OS did not give a good error message when encountering that bug!
Regards,
Ulrich