That might be a bug that appeared in C3, I only encountered that once, I hope that is now fixed in the latest beta.
To fix my project I followed this from Dop2000.
Looks like some files in the project are missing or corrupted.
Check the files mentioned in the console error messages, for example gg_anim-faling-000.png
If this file exists, try opening/saving it with a program like Paint. Or try restoring it from older backups, or replace it with another file of similar resolution. See if it makes the error message in the console to go away. Do this for other files.
I highly recommend you use Git on your project and commit every once in a while. This way you always have backup.