The messsage indicates that the file has been tampered with or improperly saved.
It’s hard to say, but it sounds like you’re using the old unsupported version 1.x of AxCrypt.
If that’s the case, you can always try to decrypt the file anyway and ignore the error. It probably won’t work, but you can try. See http://www.axantum.com/AxCrypt/Registry.html for details, it’s the TryBrokenFile setting.
If you’re using AxCrypt 2, let us know. There’s a setting there too that you can try.
But, in any case, backups are your protectoin against data loss for all data, encrypted or not. Think if you might not have a copy of the file somewhere.