It is usually a PHP setting which is unrelated to LimeSurvey.
You will need to google how to correctly configure your PHP session save path or you contact your server/webspace provider.They might be able to to figure out why sessions are not working properly.
I had to enable SSH on my hosting server then use sftp with filezilla to see the root directory and full path. I created a folder (755) named tmp outside of limesurvey then I created a php5.ini file in the html folder with the full path to the new tmp location.
If you are using Linux hosting it's the folder where your html files are. The path I posted is for my server, you would use the full path to your new tmp folder you see in the ssh sftp filezilla Remote site: address bar