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Connecting to Filezilla using implicit mode FTPS works fine. But when a directory listing or file transfer is attempted, Filezilla returns the error '521 PROT P required'.
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Filezilla server is a bit unusual in that it requires the PBSZ 0 and PROT P commands (which are mandated in explicit mode FTPS) to be called for implicit FTPS. Most servers don't. There is no RFC for implicit mode, so there is no right or wrong way.

Using SecureFileTransferClient, you'll need to call these commands immediately after logging in: 

client.executeCommand("PBSZ 0");
client.executeCommand("PROT P");