This can be caused by cookies in your browser, if this is the case it could be resolved by deleting all the stored cookies or you can delete them selectively for the directory or domain where your phpAdmin is.
This could also be caused with directory permissions or ownership issue. Find the directory where the sessions are stored in the php.ini file. It should be similar or close to this
session.save_path = "/var/lib/php/session"
When you find the directory, check the permissionsof this directory and making sure that your server has the right permissions to access it.
You can change the ownership of the directory like this
sudo chown “YourUserName”:”YourGroupName” “session save path/ php/session”
Or you can just change the permissions of the directory to world readable and writeable like this
sudo chmod 0777 “Path/php/session”