If I repeat the backup after the first error occurs, the restarted backup aborts with the following error:
[Error-Duplicati.Library.Main.Operation.BackupHandler-FatalError]: Fatal error\r\nFtpCommandException: Code: 421 Message: Client limit reached. Try again later.",
[Error-Duplicati.Library.Main.Controller-FailedOperation]: Die Operation Backup ist mit folgenden Fehler fehlgeschlagen: One or more errors occurred. (Code: 421 Message: Client limit reached. Try again later. (Code: 421 Message: Client limit reached. Try again later.) (One or more errors occurred. (Code: 421 Message: Client limit reached. Try again later.)))\r\nAggregateException: One or more errors occurred. (Code: 421 Message: Client limit reached. Try again later. (Code: 421 Message: Client limit reached. Try again later.) (One or more errors occurred. (Code: 421 Message: Client limit reached. Try again later.)))" ],
Error 421 is discussed here!
The remote device I use support 20 FTP connections simultaneously.
I have currently set the default value --asynchronous-upload-limit = 4.