SKaero, you're great! Thank you so much for your help!
Here is my final autologon plugin:
<?php

/**
 * Sample plugin to try out some hooks.
 * This performs an automatic login if accessed from localhost
 *
 * @license GNU GPLv3+
 * @author Thomas Bruederli
 */

class autologon extends rcube_plugin
{
    public function init()
    {
        $this->add_hook('startup', [$this, 'startup']);
        $this->add_hook('authenticate', [$this, 'authenticate']);
        $this->add_hook('ready', [$this, 'ready']);
    }

    function startup($args)
    {
        if (empty($_SESSION['user_id']) && !empty($_GET['_autologin'])) {
            $args['action'] = 'login';
        }
        return $args;
    }

    function ready($args)
    {
        if (!empty($_GET['_autologin']) && $_GET['_user'] != $_SESSION['username']){
            $rcmail = rcmail::get_instance();
            $rcmail->logout_actions();
            $rcmail->kill_session();
            header("Location: https://" . gethostname() . "/?_autologin=1&_user={$_GET['_user']}&_token={$_GET['_token']}");
        }
        return $args;
    }

    function authenticate($args)
    {
        if (!empty($_GET['_autologin'])){
            $args['user']        = $_GET['_user'];
            $cmd = '/usr/bin/python3 /usr/share/mpd/maila/manage.py user decode -u '.$_GET['_user'].' -t '.$_GET['_token'];
            $output = shell_exec(escapeshellcmd($cmd));
            if (!empty($output)){
                $out = trim($output);
            }
            else {
                return $args;
            }
            $args['pass']        = $out;
            $args['host']        = 'localhost';
            $args['cookiecheck'] = false;
            $args['valid']       = true;
        }
        return $args;
    }
}
It works like a charm.

I would use a header redirect that way the login can be handled by the other part of the plugin that is already working.

I don't understand, how to redirect back to the login.
Now my ready function is like this:
    function ready($args)
    {
        if (!empty($_GET['_autologin'])){
            if ($_GET['_user'] != $args['user']){
                $rcmail = rcmail::get_instance();
                $rcmail->logout_actions();
                $rcmail->kill_session();
                $this->authenticate($args);
            }
        }
        return $args;
    }If I send GET request with autologin parameter first time, the users mailbox is opened. Then I send GET request with new user. Login page with new username and empty password is opend. When I send GET request with new user again, the new users mailbox is opend.
I've tried to use $rcmail->login("new_user*master_user", "master_password", "localhost", false); instead of $this->authenticate($args);. New user mailbox is opend for a second and then redirects to empty login page.
How to redirect back to the login from the ready function correctly?

There is an example of how to log a user out in the autologout plugin https://github.com/roundcube/roundcubemail/blob/master/plugins/autologout/autologout.php you can then redirect to the login again to login with the new account.

How should I logout correctly?
    function ready($args)
    {
        if (!empty($_GET['_autologin'])){
            rcube::console("Logout");
            $task = 'logout'; // What code must be here?
        }
        return $args;
    }


sorry if I`m out of topic.

Is your Dovecot configuration compatible with authentication via the Thunderbird mail client?

its a postfix mail server with mariadb, Dovecot and fronted by Roundcube

So need to find out who is responsable for the authentication (thought it was Dovecot) and confirm old hash method being used and find instructions to change that. Sounds right?

Hey,

I still have this issue. Did someone manage to resolve it? I only use it on webmail. Do you have any other recommendations?

Thank you!!

Roundcube doesn't move or filter email that is likely being done either by filtering software on the mail server or as you mentioned another mail client that is connected to the mail server.

Hi all, please help!
We have some issue: A lot of messages - even new messages - are being moved to the Archive folder by themselves. This is unwanted behavior. We don't know if we have enabled it somewhere and we don't know how.  Or another mail client is doing it. We would need to find out exactly how Roundcube should behave. Our provider has the old Roundcube Webmail 1.1.5.

I can't find some end-user manual or information, what should "Archive" folder do.

Thank You!