Here's all the verbose output i get:
17:14:10.872 Statistics reset.
17:14:10.876 D-BUS: Service is available (org.jackaudio.service aka jackdbus).
17:14:10.964 D-BUS: JACK server is starting...
17:14:10.984 D-BUS: JACK server was started (org.jackaudio.service aka jackdbus).
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
Cannot connect to server socket err = No such file or directory
Cannot connect to server request channel
jack server is not running or cannot be started
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
JackShmReadWritePtr::~JackShmReadWritePtr - Init not done for -1, skipping unlock
Mon Jul 29 17:14:10 2019: Starting jack server...
Mon Jul 29 17:14:10 2019: JACK server starting in realtime mode with priority 10
Mon Jul 29 17:14:10 2019: self-connect-mode is "Don't restrict self connect requests"
Mon Jul 29 17:14:10 2019: Acquired audio card Audio2
Mon Jul 29 17:14:10 2019: creating alsa driver ... hw:B5|-|64|3|192000|0|0|nomon|swmeter|-|32bit
Mon Jul 29 17:14:10 2019: configuring for 192000Hz, period = 64 frames (0.3 ms), buffer = 3 periods
Mon Jul 29 17:14:10 2019: ALSA: final selected sample format for playback: 24bit little-endian in 3bytes format
Mon Jul 29 17:14:10 2019: ALSA: use 3 periods for playback
Mon Jul 29 17:14:10 2019: graph reorder: new port 'system:playback_1'
Mon Jul 29 17:14:10 2019: New client 'system' with PID 0
Mon Jul 29 17:14:10 2019: graph reorder: new port 'system:playback_2'
Mon Jul 29 17:14:12 2019: Saving settings to "/home/steam/.config/jack/conf.xml" ...
17:14:13.181 JACK connection change.
17:14:13.181 Statistics reset.
17:14:13.182 Client activated.
17:14:13.182 Patchbay deactivated.
17:14:13.183 JACK connection graph change.
Mon Jul 29 17:14:13 2019: New client 'qjackctl' with PID 26730
17:14:39.502 JACK connection graph change.
Mon Jul 29 17:14:39 2019: New client 'alsa-jack.jackP.24814.9' with PID 24814
Mon Jul 29 17:14:39 2019: Client 'alsa-jack.jackP.24814.9' with PID 24814 is out
17:14:59.568 Client deactivated.
17:14:59.602 D-BUS: JACK server is stopping...
17:14:59.624 D-BUS: JACK server was stopped (org.jackaudio.service aka jackdbus).
Mon Jul 29 17:14:59 2019: Client 'qjackctl' with PID 26730 is out
Mon Jul 29 17:14:59 2019: Stopping jack server...
Mon Jul 29 17:14:59 2019: Client 'system' with PID 0 is out
Mon Jul 29 17:14:59 2019: Released audio card Audio2
As you can see it tries to connect but in split second fails and goes out.
What i don't know is that, MC25 might be trying to connect not directly, but via alsa loopback device. This jack (alsa) output entry in MC25, did bug my eyes for a while now. Though i had no luck finding any info on jack in MC25 or any previous version whatsoever. It would be nice to get some official info on that.