Do you have the latest version (Alpha 8)? This sounds like a problem we had earlier, that got fixed with Alpha 8. Please let me know which version you are using.
The latest version can be downloaded here: https://animationpaper.com/download-pre-release/
If you did indeed use an older version, please go ahead and install the latest. Your AP file will load just fine in the new version, but hopefully you will now be able to export without any problems.
Thanks,
Niels