That is a lot of problems in one go!
Lets take them one at a time.
USB backup
This is what is probably simplest to investigate, and also the most concerning.
This means that the HMAC integrity check fails. The file looks correct, but something is not right with the file contents. If you have the database that was originally created with the backup (i.e. not from a recreate/repair command), can you look in that and see if the SHA256 hash of duplicati-b6320983bc03b40188d8dbcfb8283daec.dblock.zip.aes matches the file on disk?
If the two hashes match, then it indicates that Duplicati has written a file that it cannot read itself, which is a grave problem that needs to be investigated. Any further information that you can give on this is much appreciated.
Dropbox
This means that the data returned from Dropbox was not the file (it should start with the AES header marker, and it does not. I do not recall seeing this error with Dropbox before, but there could be a number of reasons, such as temporary glitch on Dropbox, error in the downloader, etc. If you look at the file duplicati-b8903ffb010ac41c79fdb5eebff19b4f8.dblock.zip.aes with a hex editor, the first 3 bytes should be be the characters AES. I am very curious to hear what are in those three bytes.
That does sound strange. My best guess is that the LIST command is excluding the source folder itself. The Restored files counter is applied after restoring all files and when checking the file integrity. The LIST counter is querying the database table. One potential source of this could be that some file is “restored” twice, but I have not seen this before.
Can you try this and see if it outputs 616 or 615 lines:
"c:\Program Files\Duplicati 2\Duplicati.CommandLine.exe" list * --version=1 --full-result --dbpath=C:\Us
HubiC
Die angefragte Datei existiert nicht => Die angefragte Datei existiert nicht
That indicates that the file was uploaded at some point, but now it does not exist. Can you see the duplicati-b475693647b054408a162c6542fe2ec80.dblock.zip.aes file on HubiC via the webinterface? Can you download it?
USB Passphrase error
That error message means that the file appears to be valid, but does the passphrase was not. If you have only a single file with this error, I am leaning towards some memory corruption as a possible explanation.
Can you try to decrypt the file duplicati-b6320983bc03b40188d8dbcfb8283daec.dblock.zip.aes with AESCrypt:
Box.com
That looks like a mix of the other problems, I think once we figure out what happens with the others, this will reveal/fix these as well.
I am not 100% on the context, but it looks like the data files are perhaps broken somehow (from some of the other errors?), and then the verification step sees these files that are supposed to be there, but not hence the Extra: problems.
I hope we can figure out what happens, and then come up with a solution. Luckily it seems you are only testing this, so no real data has been lost.