- Siegfried Gyuricsko found out that the curl manual said --retry would retry

on FTP errors in the transient 5xx range. Transient FTP errors are in the
  4xx range. The code itself only tried on 5xx errors that occured _at login_.
  Now the retry code retries on all FTP transfer failures that ended with a
  4xx response.

  (http://curl.haxx.se/bug/view.cgi?id=2911279)

docs/curl.1

@@ -1079,7 +1079,7 @@ timestamp.
 If a transient error is returned when curl tries to perform a transfer, it
 will retry this number of times before giving up. Setting the number to 0
 makes curl do no retries (which is the default). Transient error means either:
-a timeout, an FTP 5xx response code or an HTTP 5xx response code.
+a timeout, an FTP 4xx response code or an HTTP 5xx response code.
 
 When curl is about to retry a transfer, it will first wait one second and then
 for all forthcoming retries it will double the waiting time until it reaches