Seems reasonable and thanks for pointing out, where the error on the module appears.
The recipient's ftp server seems to throw the HELP command as the reply to entering into PASV.
I compared the output from the same script, which runs OK on old server, and on new server

OLD server

>>> TYPE I
<<< 200 Type set to I
>>> PBSZ 0
<<< 200 PBSZ Command successful.
>>> PROT P
<<< 200 PROT Command successful.
>>> PASV
<<< 227 Entering Passive Mode (XXX,XXX,XXX,XXX,23,237)
>>> ALLO 5
<<< 202 ALLO command ignored.
>>> STOR test
<<< 150 Opening data connection for test.
<<< 226 Transfer complete.
>>> QUIT
<<< 221 Goodbye.
[download]

NEW server

>>> TYPE I
<<< 200 Type set to I
>>> PBSZ 0
<<< 200 PBSZ Command successful.
>>> PROT P
<<< 200 PROT Command successful.
>>> PASV
<<< 227 Entering Passive Mode (XXX,XXX,XXX,XXX,23,243)
>>> HELP
<<< 214- The following commands are recognized (* =>'s unimplemented).
<<<    USER    PORT    STOR    MSAM*   RNTO    NLST    MKD    CDUP
<<<    PASS    PASV    APPE    MRSQ*   ABOR*   SITE    XMKD*  XCUP*
<<<    ACCT*   TYPE    MLFL*   MRCP*   DELE    SYST    RMD    STOU
<<<    SMNT*   STRU    MAIL*   ALLO    CWD     STAT    XRMD*  SIZE
<<<    REIN    MODE    MSND*   REST*   XCWD*   HELP    PWD    MDTM*
<<<    QUIT    RETR    MSOM*   RNFR    LIST    NOOP    XPWD*  FEAT
<<<    AUTH    PBSZ    PROT    CCC
<<< 214 Direct comments to sysadm.
<<+ 214 The ALLO command is supported.
>>> ALLO 5
<<< 202 ALLO command ignored.
>>> STOR test
<<+ 555 Can't read command channel socket:
>>> QUIT
<<+ 555 Can't read command channel socket:
[download]

The further attempts to reveal the truth lead me to the strace output:

open("test", O_RDONLY)                  = 4
ioctl(4, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fff91df0b70) = -1 ENOTTY (I
+nappropriate ioctl for device)
lseek(4, 0, SEEK_CUR)                   = 0
fstat(4, {st_mode=S_IFREG|0777, st_size=5, ...}) = 0
fcntl(4, F_SETFD, FD_CLOEXEC)           = 0
write(2, ">>> PBSZ 0\n", 11>>> PBSZ 0
)            = 11
write(3, "\27\3\1\0\30Y\334L\2637>O\211\225C\313vG\302\233\311/\354\34
+\n\375\26JH", 29) = 29
read(3, "\27\3\1\0.", 5)                = 5
read(3, "|\3\256\307\305\26\3N\177\270\255\333\323\365a,\350\n9\234&\3
+52\274\316\320{\2264\301y\204\321"..., 46) = 46
write(2, "<<< 200 PBSZ Command successful."..., 33<<< 200 PBSZ Command
+ successful.
) = 33
write(2, ">>> PROT P\n", 11>>> PROT P
)            = 11
write(3, "\27\3\1\0\30<\203,c\325/\26\211\27*\246Q6\313,F\301\251\247l
+\327\227T\236", 29) = 29
read(3, "\27\3\1\0.", 5)                = 5
read(3, "\316>\361\207&%\350\351z\301R\234Z\227k\257\370<k9E\267P\225\
+r\300R\6\331\374\273\227"..., 46) = 46
write(2, "<<< 200 PROT Command successful."..., 33<<< 200 PROT Command
+ successful.
) = 33
write(2, ">>> PASV\n", 9>>> PASV
)               = 9
write(3, "\27\3\1\0\26\207\31Pp!\301R\17\31\2600\275\203\326\34\233Z\2
+1\225|\r|", 27) = 27
read(3, "\27\3\1\0A", 5)                = 5
read(3, "\263+\7(\272Fi\322\356\24 ;i\246\213\374\253\346\206\23\n'\24
+4\366c\222\306wkCF\301"..., 65) = 65
write(2, "<<< 227 Entering Passive Mode (1"..., 52<<< 227 Entering Pas
+sive Mode (XXX,XXX,XXX,XXX,23,212)
) = 52
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 5
ioctl(5, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fff91df0d60) = -1 EINVAL (I
+nvalid argument)
lseek(5, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
ioctl(5, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fff91df0d60) = -1 EINVAL (I
+nvalid argument)
lseek(5, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
fcntl(5, F_SETFD, FD_CLOEXEC)           = 0
connect(5, {sa_family=AF_INET, sin_port=htons(6100), sin_addr=inet_add
+r("XXX.XXX.XXX.XXX")}, 16) = 0
stat("test", {st_mode=S_IFREG|0777, st_size=5, ...}) = 0
stat("test", {st_mode=S_IFREG|0777, st_size=5, ...}) = 0
[download]

What do you think, is it failing to open the file 'test'? and this fails the "command channel"?