Correction.
Selection a mirror from menu item "Choose a mirror of the Debian
archive" -- above "Install the base system" -- doesn't succeed. Never.
Even after a base system is installed it failes.
However, with the next item "Configure the package manager" again comes
the question if I want to choose a mirror. Saying "Yes" here does work,
even though the menu dialog sequence seems to be exactly the same as
"Choose a mirror of the Debian archive" -- with the BIG difference that
it works, where as the separate menu item from the installer does not.
Maybe this behaviour is normal... then I appologize... I had forgetten.
I performed my last Debian netinst over a year ago.