I was recently working in offline mode when the project preview stopped working.
The F12 console error (below) said it couldn't fetch the Timer behaviour.
Originally, I was working on this file in online mode, then moved to offline. The project preview worked fine, until I added a new behaviour to a sprite.
I've never seen this before - is there a way around this? I want to keep working on the flight back, but this has got me a bit worried I won't be able to.