Did you have the same problem?
Well, do make sure that adminer can access the session folder or if there's a restriction such as openbase dir.(php open_basedir).
To check you may need to create a temporary file called 000info.php with the following contents.
You can use your favorite FTP program or the File Manager within your control panel to create and update it.
Then point your browser to your-site.com/000info.php
Checking session.save_path php setting
We're using 000info.php because many people will try to access info.php or phpinfo.php and see your php settings. The less people know about your site and configuration the better. After you're done delete the file.
session.save_path usually points to /var/lib/php/sessions, however it may have been modified.
Checking session.open_basedir php setting
This may be empty or contain multiple folders. With this setting php can't open files outside of the whitelisted folders.
What is adminer?
Adminer is a full-featured database management tool written in PHP. It consist of a single file ready to deploy to the target server. Adminer is available for MySQL, MariaDB, PostgreSQL, SQLite, MS SQL, Oracle, Elasticsearch, MongoDB and others via plugin