It’s not entirely easy to know what has happened, but the error message is not from AxCrypt, it’s from Microsoft Word I believe.
Can you perhaps try to just decrypt the file, and see if that works?
If it was an existing file, and you have a backup via Time Machine, you can always restore it from there.å
We’ve had no previous reports of file corruption in the Mac like this, so something else has happened.
Please remember – you cannot open an encrypted file from *within* Word! You have to open it via AxCrypt (or from Finder, which will invoke AxCrypt), or decrypt it first using AxCrypt.