Can you create a new project and then try importing a puppet into that?
It gives the same error message.
What if you launch the app holding down Shift, then try?
Same result. It also gives me an additional error message,
bad argument #1 to "draw image"
Do you see similar errors when you click "Start Here!", "New Puppet in Photoshop", or "New Puppet in Illustrator". All of those commands are found in the Welcome screen.
Can you try "File>New Empty Puppet", then delete the puppet?
Finally, it sounds like your problem is more general, but feel free to zip a bad project with puppets and email them to me at firstname.lastname@example.org. Please change the file extension to "zap", otherwise I might never get it(zips are blocked).
Seems like the workspace had gotten jacked up and resetting it via the "Start Here!" got me back to a working Character Animator!