This depends on the version of AxCrypt you’re using – but it also signifies a serious problem with the file. The meaning is essentially that the file has been modified after it was encrypted. That’s not good.
You can’t really ‘fix it’. With AxCrypt 1.x you can chose to ignore it, but no guarantees are given as to the correctness of the decryption. Go to http://www.axantum.com/ and check the section on registry settings for instructions how to instruct AxCrypt to ignore the error.