Number of (ghost) sessions grow and never cleared

I setup a new VPS with ubuntu 18.04.4, including virtualmin/usermin. In auth.log I see a lot of

    su[12936]: Successful su for domain by root
    su[12936]: + ??? root:domain
    systemd-logind[148]: New session c315 of user domain .
    su[12936]: pam_unix(su:session): session opened for user domain by (uid=0)
    su[12936]: pam_unix(su:session): session closed for user domain

in syslog, I see a lot of

    systemd[1]: Started Session c314 of user domain.
    systemd[1]: Started Session c315 of user domain.

domain is the user of my virtual server defined in the VPS. c314/c315 increased by 1 each time, every 5 minutes (see explanation below). When running `loginctl list-sessions` those sessions number grow as "active=yes" and "state=closing" and never disappear from the list.

a random session as an example:

    loginctl session-status c315
    c315 - domain (1000)
               Since: Sat 2020-02-08 20:27:08 UTC; 23h ago
              Leader: 12936
                 TTY: ???
              Remote: user root
             Service: su; type tty; class user
               State: closing
                Unit: session-c315.scope

I did a simple test: logged in with ssh, logged out (exit) and re-logged in. I saw 2 sessions in loginctl list-sessions and the previous one, which I obviously closed, never disappeared from the list.

as a "bonus": Virtualmin apparently open/close a session for the virtual server user to do its stuff causing the number of ghost sessions to grow forever...

I posted this question in server fault, askubuntu, ubuntuforums and virtualmin - no one knew what to tell me. I have no idea what/where to look for... Please advice :) Thanks in advance.

FWIW: Someone from virtualmin forum told me that he had a similar problem with debian 9 and it was fixed in debian 10.

After a boot I see this, maybe related?

Feb 13 06:39:48 domain systemd[1]: Starting User Manager for UID 1000...
Feb 13 06:39:48 domain systemd[636]: Failed to create /user.slice/user-1000.slice/user@1000.service/init.scope control group: Permission denied
Feb 13 06:39:48 domain systemd[636]: Failed to allocate manager object: Permission denied
Feb 13 06:39:48 domain systemd[1]: user@1000.service: Failed with result 'protocol'.
Feb 13 06:39:48 domain systemd[1]: Failed to start User Manager for UID 1000