Mouse disappears on virtualbox windows for ubuntu 18.04 server

Here is the my machine’s current setup.

windows 8.1 64bit
virtualbox 6.0.24r
ubuntu 18.04.5 server-amd64

When I move the mouse over the ubuntu windows, the mouse disappears.
I have tried the following methods but none of them works.

1> change mouse from 'PS/2 Mouse' to 'USB Tablet'
2> toggle Disabling Mouse Integration
3> sudo rmmod psmouse; sudo modprobe psmouse
4> installed guest addition
5> disabled Enable 3D Acceleration

What else should I try?

Thank you