Cookie issue (session problems)

[eluser]Unknown[/eluser]
Hi,
I have web app which users can log into and add their photos, everything is working fine in most browsers, but in Opera we have a big issue with cookies. Sometimes users can't log into, and sometimes can't logout. Of course after deleting cookies it works, but deleting cookies for most users is too difficult. Does Opera need weird cookie handle or something? Web app is based on Code Igniter framework, and sessions are handle by cookies. Cookies are encrypted in CodeIgniter with encryption key. I thought it may be a problem, but it works daily, only sometimes cookie crash, and users can't log in or log out.
Does anyone have an idea what can cause this strange behaviour of Opera?
I have tested almost everything, I tried different version of Opera, from 9.x to 10.x and users still report session issues in Opera.

Address of web app: http://www.flog.pl
Code Igniter version: 1.5.3
Cookie name: blog_session
Average cookie size: ~2KB

Thanks for help,
Michael.