Minor Track Ball Issues

Greetings, people!

I have been working on rotating a camera with track ball script for the past one week. Generally things are going okay, the camera moves left/right/up/down along with the mouse quite nicely. But the problem is, when the camera is rotated to another side of the game arena, it starts to go upward when the mouse goes downward.

Apparently my rusty trigonometry is not helping at all in figuring out what is wrong (I am sure the solution to this is really simple, but I cannot get it to work). I have attached a bare minimal sample Unity project that shows this problem, and I'm confident that you experts can help me with the script.

Thanks in advance for any help that I am getting! :)

Regards, Ben.