I suspect holding option while refreshing might fix it (holding option triggers the compiled scene and cached data to be deleted and regenerated).
However, in the interests of preserving some forensic data, could you instead do this:
1. Reveal the project folder in Explorer (File menu)
2. Open the Ch Data folder
3. Rename the temp.noindex folder (add a .old to the end or whatever)
4. Go back to Character Animator and refresh the scene.
If the error no longer occurs at step 4, I'd be curious to inspect the differences between files in the old temp.noindex folder and the newly regenerated one. My guess is that an error caused one of the files to be written incopmpletely.
If step 3 above presents an error about files being in use, you may need to quit/relaunch the app to perform that step.