tasksel fails

[Markus <alpha@violine.ch>]

Just did a fresh install and started with debian-8.0-alpha-CD-1.iso

Everything went really smooth until tasksel.
No matter what additional packages i have selected, tasksel fails with
tasksel: apt-get failed (100)

Then i did clean up apg-get:
apt-get clean
cd /var/lib/apt
mv lists lists.old
mkdir -p lists/partial
apt-get clean
apt-get update

and with "apt-get update" i got this message:
Reading package lists... Done
W: GPG error: http://ftp.debian-ports.org unstable InRelease: The following
signatures couldn't be verified becaus         e the public key is not
available: NO_PUBKEY A53AB45AC448326E
W: GPG error: http://ftp.debian-ports.org unreleased InRelease: The
following signatures couldn't be verified beca         use the public key is
not available: NO_PUBKEY A53AB45AC448326E

how to workaround this ?
may this be there reason for tasksel to fail ?

thanks for feedback