Thank you! 
It hasn’t been reported but I’ve seen this error occasionally when the MIDI ports are already in use because Elektroid opens the MIDI ports exclusively. For the same reason, only one instance of Elektroid can be used with a given device at once.
Try killall -9 elektroid before restarting the GUI in case it helps and feel free to create a GitHub issue. BTW, run it with -vvv to produce a verbose ouput.
I’ll think about the Flatpack packaging but it’s already too much work.
One last thing. I’ve developed Overwitch (see Overwitch, a JACK client for Overbridge devices, AKA Overbridge for Linux), which is a Overbridge implementation for JACK. Version 1.0 is about to be released but AR MKII is the only device that has not been reported to work. Could you give it a try? It would be really great. TIA.