无法用普通帐号登录了，可以用root登录

cat /var/log/auth.log

Feb 9 17:39:01 hwy CRON[3404]: pam_unix(cron:session): session opened for user root by (uid=0)
Feb 9 17:39:01 hwy CRON[3404]: pam_unix(cron:session): session closed for user root
Feb 9 17:40:01 hwy CRON[3459]: pam_unix(cron:session): session opened for user smmsp by (uid=0)
Feb 9 17:40:01 hwy CRON[3459]: pam_unix(cron:session): session closed for user smmsp
Feb 9 17:40:02 hwy polkitd(authority=local): Unregistered Authentication Agent for unix-session:12 (system bus name :1.116, object path /org/freedesktop/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8) (disconnected from bus)
Feb 9 17:40:36 hwy CRON[641]: pam_unix(cron:session): session opened for user root by (uid=0)
Feb 9 17:40:36 hwy sshd[619]: Server listening on 0.0.0.0 port 22.
Feb 9 17:40:36 hwy sshd[619]: Server listening on :: port 22.
Feb 9 17:40:36 hwy CRON[641]: pam_unix(cron:session): session closed for user root
Feb 9 17:40:37 hwy systemd-logind[624]: New seat seat0.
Feb 9 17:40:37 hwy systemd-logind[624]: Watching system buttons on /dev/input/event4 (Power Button)
Feb 9 17:40:37 hwy systemd-logind[624]: Watching system buttons on /dev/input/event11 (Video Bus)
Feb 9 17:40:37 hwy systemd-logind[624]: Watching system buttons on /dev/input/event3 (Power Button)
Feb 9 17:40:47 hwy lightdm: pam_unix(lightdm-greeter:session): session opened for user lightdm by (uid=0)
Feb 9 17:40:47 hwy systemd-logind[624]: New session c1 of user lightdm.
Feb 9 17:40:48 hwy systemd: pam_unix(systemd-user:session): session opened for user lightdm by (uid=0)
Feb 9 17:40:51 hwy sshd[619]: Received SIGHUP; restarting.
Feb 9 17:40:51 hwy sshd[619]: Server listening on 0.0.0.0 port 22.
Feb 9 17:40:51 hwy sshd[619]: Server listening on :: port 22.
Feb 9 17:41:01 hwy lightdm: pam_unix(lightdm-greeter:session): session closed for user lightdm
Feb 9 17:41:01 hwy lightdm: pam_unix(lightdm:session): session opened for user lubantu by (uid=0)
Feb 9 17:41:01 hwy systemd-logind[624]: New session 2 of user lubantu.
Feb 9 17:41:01 hwy systemd: pam_unix(systemd-user:session): session opened for user lubantu by (uid=0)
Feb 9 17:41:01 hwy lightdm: pam_unix(lightdm-greeter:session): session opened for user lightdm by (uid=0)
Feb 9 17:41:01 hwy systemd-logind[624]: New session c2 of user lightdm.
Feb 9 17:41:04 hwy systemd-logind[624]: Removed session c1.
Feb 9 17:41:10 hwy lightdm: pam_unix(lightdm-greeter:session): session closed for user lightdm
Feb 9 17:41:10 hwy lightdm: pam_unix(lightdm:session): session opened for user root by (uid=0)
Feb 9 17:41:10 hwy systemd-logind[624]: New session 3 of user root.
Feb 9 17:41:10 hwy systemd: pam_unix(systemd-user:session): session opened for user root by (uid=0)
Feb 9 17:41:16 hwy polkitd(authority=local): Registered Authentication Agent for unix-session:3 (system bus name :1.24 [lxpolkit], object path /org/freedesktop/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8)
Feb 9 17:43:01 hwy systemd-logind[624]: Removed session 2.
Feb 9 17:43:16 hwy lightdm: pam_unix(lightdm:session): session closed for user root
Feb 9 17:43:16 hwy polkitd(authority=local): Unregistered Authentication Agent for unix-session:3 (system bus name :1.24, object path /org/freedesktop/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8) (disconnected from bus)
Feb 9 17:43:16 hwy lightdm: pam_unix(lightdm-greeter:session): session opened for user lightdm by (uid=0)
Feb 9 17:43:16 hwy systemd-logind[624]: New session c3 of user lightdm.
Feb 9 17:43:19 hwy systemd-logind[624]: Removed session c2.
Feb 9 17:43:26 hwy lightdm: pam_unix(lightdm-greeter:session): session closed for user lightdm
Feb 9 17:43:26 hwy lightdm: pam_unix(lightdm:session): session opened for user lubantu by (uid=0)
Feb 9 17:43:26 hwy systemd: pam_unix(systemd-user:session): session opened for user lubantu by (uid=0)
Feb 9 17:43:26 hwy systemd-logind[624]: New session 4 of user lubantu.
Feb 9 17:43:27 hwy lightdm: pam_unix(lightdm-greeter:session): session opened for user lightdm by (uid=0)
Feb 9 17:43:27 hwy systemd-logind[624]: New session c4 of user lightdm.
Feb 9 17:43:29 hwy systemd-logind[624]: Removed session c3.
Feb 9 17:43:32 hwy lightdm: pam_unix(lightdm-greeter:session): session closed for user lightdm
Feb 9 17:43:32 hwy lightdm: pam_unix(lightdm:session): session opened for user root by (uid=0)
Feb 9 17:43:32 hwy systemd-logind[624]: New session 5 of user root.
Feb 9 17:43:34 hwy polkitd(authority=local): Registered Authentication Agent for unix-session:5 (system bus name :1.48 [lxpolkit], object path /org/freedesktop/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8)

root 和 lubuntu 是仅有的两个账户，查看日志信息也没有看到任何线索。

viewtopic.php?p=3185237#p3185237
試試 使用 root 登入
創建一個新的用戶名稱
然後改用新的用戶名稱 登入

如果需要賦予新的用戶名稱 使用 sudo 權限
只要再把新的用戶名稱 加入 sudo 群組

建議
儘量避免使用 root 登入 除非你已經很熟悉權限配置

什么叫“无法用普通帐号登录”，你敢不敢把现象描述清楚一点？
是说你账号密码错误？没有任何反应？登录后又出错退出到登录界面？
建议你先去读一下《提问的智慧》

症状：输入正确的帐号，密码；登录后立即退出，重新返回登录框

ls -al /home/lubuntu/.Xauthority
发现权限是 root root
chown lubuntu:lubuntu /home/lubuntu/.Xauthority

reboot
解决了

我记得好几年前在12.04就遇到过这个bug……没想到现在还会出现……