Download fails immediatly with "connection lost"

I am trying to connect to an esp32 which is:

only passive
not encrypted
allows only one connection at the time

via ftp. LIST seems to work, i also can rename and duplicate files. when i try to download a file via the gui the connection immediatly gets lost.

Any hint?

i found the checkbox "download in background".
Jesus, i should finally go out and buy a rubber duck.