Go to my home directory.
Read the X's session log file for some clue.
$ cat .xsession-errors /etc/gdm3/Xsession: Beginning session setup... localuser:kianmeng being added to access control list openConnection: connect: No such file or directory cannot connect to brltty at :0 Script for scim started at run_im. Script for auto started at run_im. Script for default started at run_im. mkdtemp: private socket dir: Permission denied
Googled around, it seemed that this is quite a common issue. To confirm my finding.
$ ls -ld /tmp drwxr-xr-x 17 root root 20480 Jun 15 12:42 /tmp/
Applied the fix.
$ sudo chmod a+wt /tmp
What is the actual root cause? Last I remembered, I was restarting the machines repeatedly without login to the machine, for some BIOS settings tweaking. Most likely that's reason.