lftp - "Delaying before retry"

[hstoellinger]

Hello,
I am trying to upload a large number of files to a server using lftp like this:
lftp -u user@host,pw host
mirror -R localDir remoteDir
After entering the mirror-command, I keep getting the message "Delaying before retry " .
This message ist received repeatedly, the delay starting at 29 seconds and getting
bigger and bigger at every recursion.
I wonder what's wrong.
Thanks for any hints
H. Stoellinger, Salzburg
P.S.: Server runs Debian,client Kubuntu (Ubuntu 18.10-based).