connection gets dropped while synchronizing folders

hello there,

my problem is the following: i have an ftp server running on a win2k8 r2 server. it generally works but when i synchronize folders the connection gets dropped after around 20-30 seconds.
there is no problem with short connections though. few files (tried it with thre ~2 mb files) get transferred and synchronized without any problems.
the problem also does not seem to be on the server side (!!).
when i disable my _local_ firewall it works!
i put winscp on my local exception list of the firewall but that did not help. until now only deactivating the firewall completely seems to be enough to make synchronization work.
any idea what could cause this issue and which settings i should use on my firewall?

thanks in advance!

winscp version 5.5.6 4746
client: windows 8.1
server: windows server 2008r2
ftp/ftps doenst matter. happens always.
automated via powershell and .net dll makes no difference to gui. happens with both.
the exact synchronize settings also make no difference. i tried it with many different options.
the error message always is:
(automated) verbindung verloren
(gui
"Verbindung zum entferneten Rechner abgebrochen
Konnte Verzeichnisinhalt nicht abrufen
Fehler beim Anzeigen des Verzeichnisses"

these are the last lines of the log:

. 2014-10-28 12:59:18.409 Rufe Verzeichnisinhalt ab...
> 2014-10-28 12:59:18.409 CWD /bild/Lieferanten/Bartscher/Friteusen/als_htm/_vti_cnf/
< 2014-10-28 12:59:18.453 250 CWD command successful.
> 2014-10-28 12:59:18.453 PWD
< 2014-10-28 12:59:18.487 257 "/bild/Lieferanten/Bartscher/Friteusen/als_htm/_vti_cnf" is current directory.
> 2014-10-28 12:59:18.487 TYPE A
< 2014-10-28 12:59:18.519 200 Type set to A.
> 2014-10-28 12:59:18.519 PASV
< 2014-10-28 12:59:18.562 227 Entering Passive Mode (xxx,xxx,xxx,xxx,229,40).
> 2014-10-28 12:59:18.562 LIST -a
. 2014-10-28 12:59:18.599 Verbindung zum entferneten Rechner abgebrochen
. 2014-10-28 12:59:18.599 Konnte Verzeichnisinhalt nicht abrufen
* 2014-10-28 12:59:18.603 (ESshFatal) **Verbindung verloren.**
* 2014-10-28 12:59:18.603 Verbindung zum entferneten Rechner abgebrochen
* 2014-10-28 12:59:18.603 Konnte Verzeichnisinhalt nicht abrufen
* 2014-10-28 12:59:18.603 Fehler beim Anzeigen des Verzeichnisses "/bild/Lieferanten/Bartscher/Friteusen/als_htm/_vti_cnf".

it does not always happen with the same directory! its sometimes a little earlier sometimes a little later