Booting the Ubuntu system the timedate service didn't work (bug?). The date was one year behind. Therefore some certificates failed and thus some connections to repositories.
I changed timedate to ntp and all works fine!
$ sudo timedatectl set-ntp no
$ sudo apt install ntp
$ ntpq -p
$ date
Problem solved:
Booting the Ubuntu system the timedate service didn't work (bug?). The date was one year behind. Therefore some certificates failed and thus some connections to repositories.
I changed timedate to ntp and all works fine!
$ sudo timedatectl set-ntp no
$ sudo apt install ntp
$ ntpq -p
$ date