If you see:
There seems to be a problem with your login session; this action has been canceled as a precaution against session hijacking. Go back to the previous page, reload that page and then try again
Then the problem is with your file system.
Check the status of the file system that holds your /tmp directory.
I bet it is full.
Clean the /tmp directory or delete unneeded files/directories in the file system and you will be good to go.