Hi Oscar,

A few of us have been having the same error. 

You may need to disable SPOOR in your project for now to get it to work until the problem is fixed. This will mean no tracking for now though.

(Some others have reported the Tutor extension being the problem. If it's your preview not working the other way to get the preview to work seems to be to delete your Build folder then re-preview)

HTH
Helen