Looks like the modem device string is empty. Things have changed in MMDVMHost recently (importantly, SerialController.* is now UARTController.*) so some variables defined in MMDVM.ini have new names. Therefore you should probably redo you MMDVM.qn file. Move your old one out of the way for safe keeping. Then, copy the latest MMDVM.ini to MMDVM.qn and edit it for your setup.
P.S., The -f option of journalctl doesn't show many lines. It probably would have been more helpful to see more from the end of the log:
sudo journalctl -u mmdvm | tail -30
will show the last 30 lines.
Tom Early