Also, this is due to ses_ files that vivvo creates in temp folder.
You need access in order to remove those files. Ask your administrator to do this if you are not on your own dedicated server.
Sometimes that folder runs out of space and cant write in that folder any more sessions.
Clear tmp/ folder from all the sess_* contents in it with
And then restart the Apache
You will be ok after this and you will be able to connect to the admin.
After this, you can create a cron in order to automatically remove the sess_ files after some period.
Because apache should remove those files after a reboot... but that fails a lot of times...
So there you go