Have you tried restarting your computer?
You could try removing Animation Composer and then starting AE.
Let us know if it works.
Are you still facing this problem? If not, let us know how you solved it. If so, please let us know so we can assist you further.
I think it happened a a couple more times, but I was unable to find a solution and/or fix it.
I'm not working on that project anymore, but I expect the project assets/files/resources could have been at fault there.
Only guessing though.
I'll let you know if I encounter it again.
Either way, /thread.