Craft 2.6 Session Issues
There are several reasons (mostly environmental) why a person would be logged out from the CP discussed in these posts (along with fixes/workarounds):

Craft constantly logging out from admin area

Migrated Server, Now Getting Logged Out Constantly

Keep on getting logged out when deploying using Capistano

Users are always logged out on front end

Load Balancing Craft CMS - Admin gets logged out instantly

I'd start by reviewing those.


  and sometimes the words 'unknown error' pop up.


My hunch is it's related to your session issues, but you can check your craft/storage/runtime/logs folder for the underlying error message.

Also related and may be helpful:

The Case of the Missing PHP Session