Failed to generate preview output

[dberman]

Hi all,
I have encountered the following error message when trying to build or preview an output(HTML5):
"Object reference not set to an instance of an object"
I noticed that many of users have encountered this error, but each time the context (and the solution) seems to be different.
Anybody have an idea how to solve this issue?
Thanks,
Daniel

[alaltenburg]

Sometimes rebooting the computer does the trick. That has worked when I have had this problem, but have no idea what causes it.