Session Timeout: 'Error 500' message appears when a user creates New Topic but his session expired.

Precondition:
The "Session Timeout" value should equal one hour. Jcommune user with permition for create New Topic should exist.
Steps to reproduce:
1. Login to the JCommune
2. Click on the Create New Topic button
3. Fill in the 'Topic title' and the 'Message conent' fields
Note: Don't click on the 'New Topic' button
4. Don't do anything for an hour(until your session is ended)
5. Click on the 'New Topic' button

Actual: 'Error 500. Server encounters an error while processing your request. Please try to refresh this page later or go to forum main page' message appears
Expected:

1. If "remember me" checkbox was checked, created topic should be posted.
2. If "remember me" was unchecked, login form is shown and after successfull login in the system Create Topic page with filled data (see step 3) is opened.