Error during session start; please check your PHP and/or webserver log file and configure your PHP installation properly. Also ensure that cookies are enabled in your browser.
session_start(): open(SESSION_FILE, O_RDWR) failed: No such file or directory (2)
session_start(): Failed to read session data: files (path: ./tmp/session)
session_start(): open(SESSION_FILE, O_RDWR) failed: No such file or directory (2)
session_start(): Failed to read session data: files (path: ./tmp/session)
Este error se produce cuando no existe la ruta o no tiene los permisos adecuados.
En Plesk (Linux) este directorio se encuentra en:
/opt/psa/admin/htdocs/domains/databases/phpMyAdmin/
Creamos los directorios (si no existen):
mkdir tmp
mkdir tmp/session
mkdir tmp/session
Y asignamos los permisos adecuados:
chmod 1733 tmp
chmod 1733 tmp/session
chmod 1733 tmp/session
Fuente:
https://ruvic.es/error-al-acceder-a-phpmyadmin-desde-plesk-onyx-con-linux/