Problem

-- Connection between Gateway to PROFTPD server , on FTPS implicit fails with:

FTP101I 0000 35303320 596f7520 6d757374 20697373 [503 You must iss].
FTP101I 0010 75652074 68652050 42535a20 636f6d6d [ue the PBSZ comm].
FTP101I 0020 616e6420 7072696f 7220746f 2050524f [and prior to PRO].
FTP101I 12:29:49 0030 54 [T].

Resolution

Adjust in the remote site under the FTP/HTTP tab for FTP Specific command section with : QUOTE PBSZ 32;QUOTE PROT P

PROTECTION BUFFER SIZE (PBSZ)
The argument is a decimal integer representing the maximum size,
in bytes, of the encoded data blocks to be sent or received during
file transfer. This number shall be no greater than can be
represented in a 32-bit unsigned integer.

https://tools.ietf.org/html/rfc2228