通过从 Ubuntu 15.10 升级到 16.04 后，do-release-upgrade我现在只能启动到 TTY 模式。启动日志显示：
[FAILED] Failed to start Simple Desktop Display Manager.
See 'systemctl status sddm.service' for details.
查看以下建议以获取更多详细信息，我看到：
sddm.service - Simple Desktop Display Manager
Loaded: loaded (/lib/systemd/system/sddm.service; enabled; vendor preset: enabled)
Active: inactive (dead) (Result: exit-code) since Tue 2016-05-24 18:35:01 CEST; 4min 58s ago
Docs: man:sddm(1)
man:sddm.conf(5)
Process: 1801 ExecStartPre=/bin/sh -c [ "$(cat /etc/X11/default-display-manager 2>/dev/null)" = "/usr/bin/sddm" ] (code=exited, status=1/FAILURE)
May 24 18:35:00 miked-Ubuntu systemd[1]: sddm.service: Failed with result 'exit-code'.
May 24 18:35:01 …