Hi,
Every time you make changes into your script the hashing function need to be changed, using FCIV(File Checksum Integrity Verifier) you can resolve this issue. After downloading this,using console window check out for the new hashing function generated after making changes in the script. You can use the command
fciv.exe -SHA1or md5 <location of your .ovf>
newly generated hashing function need to be updated in the manifest file as well.