Thank you, @lgaetz.
I disabled SmartOffice and that fixed it.
Though this is not the first time the commercial module causing issue to me this week. In FreepBX 14, I used to disable all commercial modules with [$$ Buy] next to them. When I upgraded to FreepBX 15, I had to enable the commercial modules as some were dependent on others for updates. This caused UCP not accessible. But Google helped me find a solution, so I went back and disabled all commercial modules with [$$ Buy]. Hope the quality people develop a process (from what has been reported here) to test the modules before sending them to users.
Best