======== Required information ====
- iRedMail version (check /etc/iredmail-release): 0.9.5-1
- Linux/BSD distribution name and version: Ubuntu 16.04 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MariaDB
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? yes
====

Ubuntu 16.04 includes a new MariaDB that supports plugin authentication. By default, the sql root account is set to "unix_socket" which apparently prevents using Adminer to access the sql root since Adminer is (naturally) running non-privileged.

select user,plugin from mysql.user;
+------------+-------------+
| user       | plugin      |
+------------+-------------+
| root       | unix_socket |

Fix is to remove the unix_socket option:

  update mysql.user set plugin = '' where user = 'root';
  flush privileges;

After this I can log into Adminer as the sql root.

A better solution is probably to create a new db user, grant him full privs on amavis/ired*/sogo/vmail databases, then use that in Adminer instead of sql root.