OK, so I installed this ftp server on my Android 4.2.1 tablet
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and using the option "Restrict to Default Directory"
is what's causing the problem. That is kind of security setting on ftp server, honoured by Winscp, but not by other ftp clients, notably by standard built in Windows CLI client or Filezilla, they all seem to work and provide access to other directories, as already mentioned in this thread. That's kind of hypothesis by now, didn't have chance to try to untick that option yet, but I'll let you know.