Jack server Cannot create new client on Raspbian Jessie Lite - audio

Question

I have installed Jack 2 on Raspberry Pi 3 with Raspbian Jessie Lite following the instructions from here https://github.com/iani/RPI-first-steps#step-3-get-and-install-jack-audio-drivers and added user to "realtime" group like this http://jackaudio.org/faq/linux_rt_config.html.
When I start Jack server with following command:
$ jackd -P75 -dalsa -dhw:0 -p1024 -n3 -s -r44100
everything seems fine and Jack server output is:
jackdmp 1.9.12
Copyright 2001-2005 Paul Davis and others.
Copyright 2004-2016 Grame.
Copyright 2016-2017 Filipe Coelho.
jackdmp comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
JACK server starting in realtime mode with priority 75
self-connect-mode is "Don't restrict self connect requests"
creating alsa driver ... hw:0|hw:0|1024|3|44100|0|0|nomon|swmeter|soft-mode|32bit
configuring for 44100Hz, period = 1024 frames (23.2 ms), buffer = 3 periods
ALSA: final selected sample format for capture: 24bit little-endian in 3bytes format
ALSA: use 3 periods for capture
ALSA: final selected sample format for playback: 24bit little-endian in 3bytes format
ALSA: use 3 periods for playback
But when I try to run for instance Pure Data as jack client I get this output from Pure Data:
Connect: can't connect named semaphore name = jack_sem.1000_default_system err = No such file or directory
Cannot read socket fd = 9 err = No such file or directory
CheckRes error
JackSocketClientChannel read fail
Cannot open pure_data_0 client
error: JACK: server returned status 33
while Jack server is complaining:
ClientNotify fails name = system notification = 0 val1 = 0 val2 = 0
NotifyAddClient new_client fails name = pure_data_0
Cannot notify add client
Cannot create new client
I tried to play mp3 with mplayer using jack but it seems to behave similarly. Mplayer looks like it is playing the mp3 but there's no audio output and the console output is:
Selected audio codec: MPEG 1.0/2.0/2.5 layers I, II, III [mpg123]
AUDIO: 44100 Hz, 2 ch, s16le, 160.0 kbit/11.34% (ratio: 20000->176400)
AO: [pulse] Init failed: Connection refused
[AO_ALSA] alsa-lib: pcm_hw.c:1557:(snd_pcm_hw_open) open '/dev/snd/pcmC0D0p' failed (-16): Device or resource busy
[AO_ALSA] alsa-lib: pcm_dmix.c:1022:(snd_pcm_dmix_open) unable to open slave
[AO_ALSA] Playback open error: Device or resource busy
[AO OSS] audio_setup: Can't open audio device /dev/dsp: No such file or directory
Connect: can't connect named semaphore name = jack_sem.1000_default_system err = No such file or directory
Cannot open MPlayer [939] client
Cannot read socket fd = 9 err = No such file or directory
CheckRes error
JackSocketClientChannel read fail
[JACK] cannot open server
[AO SDL] Samplerate: 44100Hz Channels: Stereo Format s16le
[AO_ALSA] alsa-lib: pcm_hw.c:1557:(snd_pcm_hw_open) open '/dev/snd/pcmC0D0p' failed (-16): Device or resource busy
[AO_ALSA] alsa-lib: pcm_dmix.c:1022:(snd_pcm_dmix_open) unable to open slave
[AO SDL] Unable to open audio: No available audio device
AO: [null] 44100Hz 2ch s16le (2 bytes per sample)
Video: no video
Starting playback...
A: 20.1 (20.0) of 269.6 (04:29.6) 2.9%
Jack server's output is similar to the prior case:
ClientNotify fails name = system notification = 0 val1 = 0 val2 = 0
NotifyAddClient new_client fails name = MPlayer [939]
Cannot notify add client
Cannot create new client
I am really new to working with sound on linux and although I have read something about linux audio architecture I'm getting lost. I tried to google all different error messages from console outputs but didn't manage to find answer. Maybe I'm missing something really basic here.
Any suggestions are appreciated.
Thanks in advance.