I am not sure exactly how you got into this situation, but the error message simply states that whatever data you are trying to decrypt has been altered in some way.
This means that the key is correct and contents can be decrypted, but parts of the file does not contain the expected contents. The most likely cause is corrupt storage.
However, Duplicati should be able to detect that the hash of the file has changed since it was uploaded, in case it is corrupted, so I would expect the original error to be related to remote storage not having correct files. If you delete the local database and then repair from that, you loose the hashes of files before upload, and you would not see it before you attempt to decrypt the files.
I would suggest renaming or removing the offending file from the remote destination and then attempting to use “purge broken files” to get back to a working state.