TO OP Sandman and GaryDave:

Read GaryDave's post, this worked for me!

"A lot of people are adding notes about spacebar and right-mouse button to solve this issue, saying it isn't a bug.

It is a bug, I've experienced it. No amount of toggling right mouse button or spacebar works. The numpad enter key to change perspective is also tied to this somehow (ever had to tap it twice to change the view?). "

I am experiencing this as well. I have tried every key combination with no success.