Do you have some other S3 downloader? If not, Duplicati has one, but it’s not very user friendly.
The idea is to get the file some other way and see what some other decrypter tool can do with it.
Duplicati includes SharpAESCrypt, which would be the closest to its use. You can see help here.
Here is a more user-friendly GUI tool if you wish to start there. Its error message will be different.
There are other harder ways to pick this apart, e.g. one can check file hash against expectations. Generally though, I’d have thought the hash would be checked before decrypting, but I’m unsure. Decryption has a cryptographic integrity check, which is what’s complaining for you — somehow.
Note test restores on the original system should use –no-local-blocks=true to prevent shortcuts… Seeing your test error try to “patch with remote file” means it didn’t optimize for that particular file, however after getting this issue fixed, it’s best practice in the future when trying to verify restores.
Your test on Windows working is odd. I’m unsure why, so will try some other tests. I have no Mac.