Also sudo su bewirkt das Gleiche.
Es kam noch eine Meldung (aber nur einmal, beim ersten Versuch:
Mar 30 21:52:08 RPI-Docker systemd-resolved[1583]: Grace period over, resuming full feature set (UDP+EDNS0) for DNS server 192.168.3.1.
Mar 30 21:52:08 RPI-Docker systemd-resolved[1583]: Using degraded feature set (UDP) for DNS server 192.168.3.1.
Mar 30 21:52:13 RPI-Docker systemd-resolved[1583]: Grace period over, resuming full feature set (UDP+EDNS0) for DNS server fd00::5e49:79ff:fe2a:56e1.
Mar 30 21:52:13 RPI-Docker systemd-resolved[1583]: Using degraded feature set (UDP) for DNS server fd00::5e49:79ff:fe2a:56e1.
Mar 30 21:52:18 RPI-Docker sudo[26635]: root : TTY=pts/0 ; PWD=/root ; USER=root ; COMMAND=/usr/bin/su
Mar 30 21:52:18 RPI-Docker sudo[26635]: pam_unix(sudo:session): session opened for user root by ubuntu(uid=0)
Mar 30 21:52:18 RPI-Docker su[26788]: (to root) ubuntu on pts/0
Mar 30 21:52:18 RPI-Docker su[26788]: pam_unix(su:session): session opened for user root by ubuntu(uid=0)
Hat also scheinbar mit DNS zu tun.