You are getting HTML errors like this in the log
<title>Requested page locked - Storage Share - Hetzner Online GmbH</title>
Which probably indicates you are overloading the storage or it is rate limiting you.
I'd try cutting --transfers right back to --transfers 4 and --checkers 4 also and see if that helps.
Can you try an upload from somewhere else - maybe you have a VPS or dedicated server you could try from?