ftp connection problem - 550 The parameter is incorrect.

I can connect to several FTP servers except one (Which is the one i bought the app for).  After connecting and attempting to list or switch directories it shows the following in the log and alerts "'Transfer Error', Access denied to remote resource":

<snip>

<= Recv header, 0000000021 bytes (0x00000015)

0000: 230 User logged in.

=> Send header, 0000000005 bytes (0x00000005)

0000: PWD

<= Recv header, 0000000031 bytes (0x0000001f)

0000: 257 "/" is current directory.

== Info: Entry path is '/'

=> Send header, 0000000009 bytes (0x00000009)

0000: CWD :21

<= Recv header, 0000000034 bytes (0x00000022)

0000: 550 The parameter is incorrect. 

== Info: Server denied you to change to the given directory

== Info: Connection #0 to host swp0001mgt.server-web.com left intact

== Info: Access denied to remote resource

I have tried leaving remote folder blank, entering / or specifying the remote folder but none work, still the same error.  I have tried different encodings (UTF8, ASCII etc, server is engligh language), active and passing etc.

Works perfectly fine on PC using Filezilla (v3.6.0.2) whether i specify remote root folder or not.

I was going to request a refund but it does look like a good app with support so i want to try this avenue first.

Thanks