Update: Problem solved. It was a server problem, not a WordPress problem.
Web hosting company, Namecheap, messed up with their Mod Security settings. When they disable Mod Security, the said problem is gone. That’s just it.
The funny thing is that this problem existed in 2014. Fast forward into 2021, it’s still not fixed, duh.
For those who have the same problem and read this, this is the step you will need to ask your hosting support.
1. Ask support to disable Mod Security. (Whitelist rule don’t help)
2. Do your wordpress settings.
3. When finish, ask support to enable Mod Security back again.
PS: Thanks George for your comment.
