obfs4proxy can't be started on Ubuntu 16.04

I use Ubuntu Studio 16.04, 4.4.0-22-lowlatency and can't use obfs4proxy when I start tor as a service.

$cat /etc/tor/torrc:

SocksPort 0
ORPort auto
BridgeRelay 1
Exitpolicy reject *:*

## CHANGEME_1 -> provide a nickname for your bridge, can be anything you like
#Nickname CHANGEME_1
## CHANGEME_2 -> provide some email address so we can contact you if there's a problem
#ContactInfo CHANGEME_2

ServerTransportPlugin obfs4 exec /usr/bin/obfs4proxy
#ServerTransportPlugin obfs3 exec /usr/bin/obfsproxy --managed

$ sudo service tor restart
$ grep warn /var/log/tor/log

May 17 13:23:17.000 [warn] Unable to send readiness to systemd: Unknown error -13
May 17 13:23:18.000 [warn] Could not launch managed proxy executable at '/usr/bin/obfs4proxy' ('Operation not permitted').

Accordingly http://tor.stackexchange.com/questions/3723/obfsproxy-problem-in-ubuntu it can be apparmor configuration problem, but there are lines like

/usr/bin/obfsproxy PUx,
/usr/bin/obfs4proxy PUx,

in /etc/apparmor.d/abstractions/tor