Plan A

$ sudo update-alternatives --config x-session-manager

Plan B

 $ sudo vim /var/lib/AccountsService/users/$USER

[User]
Language=zh_CN
Session=
FormatsLocale=zh_CN.UTF-8
Icon=/home/zen/.face
SystemAccount=false
# 改为
[User]
Language=zh_CN
Session=xfce
FormatsLocale=zh_CN.UTF-8
Icon=/home/zen/.face
SystemAccount=false