Does it disconnect consistently on the same directory?
Do you have the same problem with other FTP clients?
Can you attach a relevant snippet of server-side log?

I connect to my web site through FTP protocol, TLS explicit encryption, port 21, connection Passive mode, Optimize connection buffer size enabled (greyed), Server response timeouts 30 s, Keepalives Executing dummy..., 30 s. When copying/moving large amount of files, program starts to count disk space and after about 20 seconds it disconnects my session.

Here is the log. Please, can you give me advice, what to set in options, or is this a bug? When using No encryption, everything works fine.

...
. 2013-08-23 09:12:55.061 Getting size of directory "/web/components/com_phocadownload/views/category"
. 2013-08-23 09:12:55.061 Retrieving directory listing...
> 2013-08-23 09:12:55.061 CWD /web/components/com_phocadownload/views/category/
< 2013-08-23 09:12:55.092 250 OK. Current directory is /web/components/com_phocadownload/views/category
> 2013-08-23 09:12:55.092 PWD
< 2013-08-23 09:12:55.097 257 "/web/components/com_phocadownload/views/category" is your current location
> 2013-08-23 09:12:55.097 TYPE A
. 2013-08-23 09:12:55.128 m_pSslLayer changed state from 4 to 6
. 2013-08-23 09:12:55.128 Disconnected from server
. 2013-08-23 09:12:55.128 Could not retrieve directory listing
. 2013-08-23 09:12:55.128 Got reply 1004 to the command 2
* 2013-08-23 09:12:55.159 (ESshFatal) Lost connection.
* 2013-08-23 09:12:55.159 Disconnected from server
* 2013-08-23 09:12:55.159 Could not retrieve directory listing

Thanks,
Marian