You might get this error if you are using Memcached . It means that you have restrated the server but you have not restarted the memcached again.
For restarting the memcached, login to your account using a program like Putty and run the following command
service memcached restart
On shared server you dont have an access to "root", so your su command will not work. Now if you have to add anything or modify the settings in php.ini file, you have to create a local copy of php.ini file for your website on root.
To do this, you have to login into SSH (Secure Shell) by using a program like putty .
After you login, copy your php.ini file using cp command
cp /web/etc/php.ini /var/www/vhosts/sitename/php.ini
You might have different paths, if you dont know where is your php.ini file over the server, you can use the following command to find the file
find . -type f -name 'php.ini'
Now you can change anything in your php.ini file and save it. You can view the changes by going here admin/reports/status/php in your drupal website.
You can use the following command :
$ cat /etc/*release
You can Go to php.ini and set the memory_limit variable to some higher value like 256M or 512M In case if you dont find php.ini file on the server. You can find it here /usr/local/psa/admin/conf/ or in etc/php.ini file. Make the changes and dont forget to restart the server again. To Restart the server you have to run following command : service httpd restart on your command line like putty.