Hi Michael. It sounds like a corrupted file. AP should definitely be able to load any ap file – even though it could have been made with an earlier alpha version.
If you don’t mind, you are welcome to email me your ap file, so we can take a closer look at it. We might be so lucky that the file can be partially fixed and then will be able to load. But it depends.
If you by any chance have a backup of your file, it would be the best option to get it back that way. But if not, feel free to send it to me. My email address is the one you got your activation key from.
Crossing fingers for you getting your animation back!
Best,
Niels