DiegoM
This bug happens to me as well, several times recently. I have a Chrome with C3 installed, and no extensions - I only use it for C3. It was happening in the beta cycle at some point.
C3 freezes and I have to close without saving, which sucks.
To me, it feels like it only happens when I've imported lots frames and had the animation editor opened for a long time, maybe doing other things like deleting frames or using hotspot.
So my workaround is to just import the frames, then quickly close the animation editor, then save, then open it again and continue work in the anim editor with placing hotspots and such.
(I'm not using project folder, just c3p files.)