Connection to storage server failed -> Empty startup greeting. How Fix?

[Silver_Light]

i installed the latest ver of roundcube (Stable: v 0.8.4) in windows server 2008 - r2. 
i had a fresh installation and everything was ok about that!
sometimes (most of the time) during login i have this error: 

Connection to storage server failed

and when i opened my logs i found many errors like these : 

(soscharge.com:143) in C:\Roundcube\program\include\rcube_imap.php on line 191 (POST /?_task=login&_action=login)
[06-Dec-2012 16:41:17 +0330]: IMAP Error: Login failed for [email protected] from 5.52.157.214. Empty startup greeting (soscharge.com:143) in C:\Roundcube\program\include\rcube_imap.php on line 191 (POST /?_task=login&_action=login)

is this a bug in roundcube?   
what is the problem and how can i fix? 
i found some thread in this forum about that, but i think those were old.

thanks in adavnce

[howave]

I met the same problem