Hi, are you using the MSI installer? If not I can't understand why you'd get that error. From what I've found on Google it seems like it could be corrupt or anti-virus software interfering, but you've already tried that.
Which URL are you using to download the installer? We could then check the MD5 hash value to make sure your download it isn't corrupt. If you are using the SyncBackSE_Setup.msi V5.20 installation then the MD5 hash value of the .msi file is A359E7AA31C067889B5E3B2F5D87D17E. We've also tested it and didn't find any problem installing with it.
Which URL are you using to download the installer? We could then check the MD5 hash value to make sure your download it isn't corrupt. If you are using the SyncBackSE_Setup.msi V5.20 installation then the MD5 hash value of the .msi file is A359E7AA31C067889B5E3B2F5D87D17E. We've also tested it and didn't find any problem installing with it.