Error in editing Tasks

[Serkan TASOLUK]

After creating a task of in an existing project, I wanted to check the details one more time. After I clicked a pop-up screen appeared with a headline of OpenERP Server Error. The remaing part of the message is listed at the end of the message. I pressed OK and the details was shown. I pressed edit and added some more details. When I pressed Save button I got the same pop-up screen with same details. Also I noticed that the changes were not saved. I did it several times but I got the same result. How can I fix the problem?

Here is the remaining part of the message:

Client Traceback (most recent call last):
  File "C:\Program Files\OpenERP\Server\server\openerp\addons\web\http.py", line 204, in dispatch
  File "C:\Program Files\OpenERP\Server\server\openerp\addons\web\controllers\main.py", line 1128, in call_kw
  File "C:\Program Files\OpenERP\Server\server\openerp\addons\web\controllers\main.py", line 1120, in _call_kw
  File "C:\Program Files\OpenERP\Server\server\openerp\addons\web\session.py", line 42, in proxy
  File "C:\Program Files\OpenERP\Server\server\openerp\addons\web\session.py", line 30, in proxy_method
  File "C:\Program Files\OpenERP\Server\server\openerp\addons\web\session.py", line 103, in send

Server Traceback (most recent call last):
  File "C:\Program Files\OpenERP\Server\server\openerp\addons\web\session.py", line 89, in send
  File "C:\Program Files\OpenERP\Server\server\.\openerp\netsvc.py", line 292, in dispatch_rpc
  File "C:\Program Files\OpenERP\Server\server\.\openerp\service\web_services.py", line 626, in dispatch
  File "C:\Program Files\OpenERP\Server\server\.\openerp\osv\osv.py", line 188, in execute_kw
  File "C:\Program Files\OpenERP\Server\server\.\openerp\osv\osv.py", line 131, in wrapper
  File "C:\Program Files\OpenERP\Server\server\.\openerp\osv\osv.py", line 197, in execute
  File "C:\Program Files\OpenERP\Server\server\.\openerp\osv\osv.py", line 185, in execute_cr
  File "C:\Program Files\OpenERP\Server\server\openerp\addons\mail\mail_thread.py", line 1159, in message_get_subscription_data
  File "C:\Program Files\OpenERP\Server\server\openerp\addons\mail\mail_thread.py", line 140, in _get_subscription_data
KeyError: u'Sorun Engellendi'