Failed to connect to socket /var/run/dbus/system_bus_socket

[Peter Reiser]

 ProFtp throws this error after each disconnect of a ftp user:

proftpd: pam_systemd(proftpd:session): Failed to connect to system bus: Failed to connect to socket /var/run/dbus/system_bus_socket: No such file or directory

After I disable RequireValidShell in proftpd.conf it vanishes.
BUT my users have a valid shell, according to etc/shell. proftpd user does not have a shell of course. so for now I have disabled RequireValidShell.

The file /var/run/dbus/system_bus_socket DOES exist and proftg is in the group nogroup:

-rw-r--r-- 1 root root 6 Jan 20 15:00 pid
srwxrwxrwx 1 root root 0 Jan 20 15:00 system_bus_socket

of course system_bus_socket is owned by root, but what do i have to change, i dont care bout that file.