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Thank you for advises rncbc
I enabled your force16bits, 3 periods/buffer and disable d-bus support advises, and I also enabled verbose mode:
ps ax|grep jack now provides a jackd feedback:
16961 ? SLsl 0:00 /usr/bin/jackd -v -P89 -p512 -dalsa -r48000 -p256 -n3 -S -Xseq -D -Chw:1,0 -Phw:1,0 -i2 -o2

And qjackctl messages displays (some lines translated):
...
09:10:33.897 Patch Bay disabled.
09:10:33.901 Reset stats.
09:10:33.913 Loading ALSA connections.
Cannot connect to server socket err = No such file or folder
Cannot connect to server request channel
jack server is not running or cannot be started
09:10:33.921 Changing ALSA connections graphic.
09:10:41.165 JACK starts...
09:10:41.166 /usr/bin/jackd -v -P89 -p512 -dalsa -r48000 -p256 -n3 -S -Xseq -D -Chw:1,0 -Phw:1,0 -i2 -o2
Cannot connect to server socket err = No such file or folder
Cannot connect to server request channel
jack server is not running or cannot be started
09:10:41.188 JACK started with PID=16989.
no message buffer overruns
no message buffer overruns
no message buffer overruns
jackdmp 1.9.10
...
JACK server starting in realtime mode with priority 89
Jack: JackPosixThread::StartImp : create non RT thread
Jack: JackPosixThread::ThreadHandler : start
Jack: capture device hw:1,0
Jack: playback device hw:1,0
Jack: apparent rate = 48000
Jack: frames per period = 256
Jack: JackDriver::Open capture_driver_name = hw:1,0
Jack: JackDriver::Open playback_driver_name = hw:1,0
Jack: Check protocol client = 8 server = 8
Jack: JackEngine::ClientInternalOpen: name = system
Jack: JackEngine::AllocateRefNum ref = 0
Jack: JackPosixSemaphore::Allocate name = jack_sem.1000_default_system val = 0
Jack: JackEngine::NotifyAddClient: name = system
Jack: JackGraphManager::SetBufferSize size = 256
Jack: JackConnectionManager::DirectConnect first: ref1 = 0 ref2 = 0
Jack: JackGraphManager::ConnectRefNum cur_index = 0 ref1 = 0 ref2 = 0
Jack: JackDriver::SetupDriverSync driver sem in flush mode
audio_reservation_init
Acquire audio card Audio1
creating alsa driver ... hw:1,0|hw:1,0|256|3|48000|2|2|nomon|swmeter|-|16bit
configuring for 48000Hz, period = 256 frames (5.3 ms), buffer = 3 periods
ALSA: final selected sample format for capture: 16bit little-endian
ALSA: use 3 periods for capture
ALSA: final selected sample format for playback: 16bit little-endian
ALSA: use 3 periods for playback
Jack: JackSocketServerChannel::Open
Jack: JackServerSocket::Bind : addr.sun_path /dev/shm/jack_default_1000_0
Jack: JackSocketServerChannel::BuildPoolTable size = 1
Jack: JackEngine::Open
Jack: JackClientSocket::Connect : addr.sun_path /dev/shm/jack_default_1000_0
Jack: JackEngine::ClientInternalOpen: name = freewheel
Jack: JackEngine::AllocateRefNum ref = 1
Jack: JackPosixSemaphore::Allocate name = jack_sem.1000_default_freewheel val = 0
Jack: JackEngine::NotifyAddClient: name = freewheel
Jack: JackDriver::ClientNotify ref = 1 driver = system name = freewheel notify = 0
Jack: JackDriver::ClientNotify ref = 0 driver = freewheel name = system notify = 0
Jack: JackConnectionManager::DirectConnect first: ref1 = 1 ref2 = 1
Jack: JackGraphManager::ConnectRefNum cur_index = 0 ref1 = 1 ref2 = 1
Jack: JackDriver::SetupDriverSync driver sem in flush mode
Jack: JackGraphManager::SetBufferSize size = 256
Jack: JackAlsaDriver::Attach fBufferSize 256 fSampleRate 44100
Jack: JackEngine::PortRegister ref = 0 name = system:capture_1 type = 32 bit float mono audio flags = 22 buffer_size = 256
Jack: JackGraphManager::AllocatePortAux port_index = 1 name = system:capture_1 type = 32 bit float mono audio
Jack: JackConnectionManager::AddOutputPort ref = 0 port = 1
Jack: JackEngine::ClientNotify: no callback for notification = 9
Jack: JackEngine::ClientNotify: no callback for notification = 9
...

I my first steps I thought I was stuck in the issue as ubuntu detected the box as "Digital" I/O (S/PDIF) and I believed from a previous very old life (before Internet era) that s/pdif==44k1, but wikipedia teaches me that s/pdif also supports 48k and 96k.

As my issue doesn't seem to be related to qjackctl, please could you advise further by telling me where you would dig and/or ask help so I can do it myself:
Korg, to know if the device can work at 48k samplerate?
Jamulus, the app that requires 48k, asking if it is possible to accept 44k1?
ALSA, for possible better support of the device?
DIY electronics forum to know if it is ever possible to tweak to change some hardware specs of the box or some transparent usb adapter that could change samplerate from 44k1 to 48k?

Thank you once more