Backups corrupt. How to repair? How can it happen?

I assume type is the command you are using, so the file really starts with some garbage data. Not sure if this is related to Dropbox

Yeah, that does really sound bad. If the files are not accessible, showing them in any listing is really a problem. Since the WebUI does not show them, I think the server is somehow aware of a file problem, but this is not reflected with WebDAV.

If you use something like --verbose you can see the names of all files restored. If you can somehow match this with the output of LIST maybe we can get find the extra file, and that can reveal the problem.

For all these problems, it appears that something is changing the file contents, hence my suggestion for memory error. But I agree with that rate of problems, your machine would not work if it had memory issues.

Are you by any chance running some kind of antivirus or other backup software that would fiddle with the temporary file folder?

I am guessing that something is reading/writing/messing with the temporary files, which causes the problems we see here.

Due to the way Duplicati works (write file, compute hash, store hash, upload) it is possible to trash the temporary file after it has been written, and this will cause Duplicati to record a “valid” hash, but upload a broken file.

This could also explain the test errors you see, if the downloaded file is modified in the temp folder, then the hash computation fails, giving the error you report.