I couldn’t get Jack to start on a RPi 5 with a HifiBerry (DAC+ADC) soundcard while everything was working fine on a RPi 4.
On the RPi 5 Jack was failing during the ALSA setup. It turns out that adding -i 2 -o 2 to the Jack command line in /etc/jackdrc solves the issue.
The Jack setup via the patchbox command may need to have an extra step to define the number of inputs and outputs used, I spent a while playing with the sound card configuration in /boot/firmware/config.txt before realising that the issue was actually coming from the Patchbox setup.
Is there a more official way of filing bug reports/feature requests outside this forum?