Levent, Could you please share the softphone.log file while the white screen is happening? You can email it to
jabber-dev-support@cisco.com The log can be found in below location
Windows XP C:\Documents and Settings\{username}\Local Settings\Application Data\softphone.log
Windows 7/Vista C:\Users\{username}\Application Data\Local\softphone.log
Mac OS X /Users/{username}/Library/Application Support/softphone.log
Thanks, Howard