FTP error only in non-development builds

Hi all,

I am making an app that, among other things, uploads data to an FTP server. Part of this involves creating a new directory on the server. This works fine in the editor and in developmentbuilds on android - the code runs and the directory appears on the server. However any attempt to do this in a non-development android build fails to create the directory and throws this exception:

mono-io-layer-error (10013)

The code in question throwing the exception looks like this:

         try
         {
             if (!directoryExits)
             {
                 FtpWebRequest ftpRequest = (FtpWebRequest)WebRequest.Create(dirToCreate);
                 ftpRequest.Credentials = new NetworkCredential(userName, password);
                 ftpRequest.UseBinary = true;
                 ftpRequest.UsePassive = true;
                 ftpRequest.KeepAlive = true;
                 ftpRequest.Method = WebRequestMethods.Ftp.MakeDirectory;
                 FtpWebResponse ftpResponse = (FtpWebResponse)ftpRequest.GetResponse();
                 ftpResponse.Close();
             }
         }
         catch (Exception ex)
         {
             result = "Error creating dir " + dirToCreate + " : " + ex.Message;
         }

Searching online it looks like this error is to do with a malformed URL but, like I said, it works correctly in debug builds and in the editor, so I find this hard to believe. Still the url (the dirToCreate variable) looks like this:

*ftp://xxx.xxx.xxx.xxx:xx/volume(sda1)/Camera*

Does anyone know what might cause this?