John,
maybe there's some progress here. I added opendv to the dialout group with:
sudo adduser -G dialout opendv

then the service starts OK for awhile:
john@ubuntu1:~$ sudo systemctl status ambeserver@...
● ambeserver@... - DV3000 Daemon
     Loaded: loaded (/lib/systemd/system/ambeserver@.service; enabled; vendor preset: enabled)
     Active: active (running) since Fri 2020-04-24 21:05:21 EDT; 33s ago
    Process: 87701 ExecStartPre=/bin/chgrp dialout /dev/ttyUSB0 (code=exited, status=0/SUCCESS)
    Process: 87710 ExecStartPre=/bin/chmod 0660 /dev/ttyUSB0 (code=exited, status=0/SUCCESS)
   Main PID: 87711 (AMBEserver)
      Tasks: 1 (limit: 2170)
     Memory: 416.0K
     CGroup: /system.slice/system-ambeserver.slice/ambeserver@...
             └─87711 /usr/sbin/AMBEserver -p 2460 -i /dev/ttyUSB0 -s 460800

Apr 24 21:05:21 ubuntu1 systemd[1]: Starting DV3000 Daemon...
Apr 24 21:05:21 ubuntu1 systemd[1]: Started DV3000 Daemon.
Apr 24 21:05:21 ubuntu1 AMBEserver[87711]: AMBEserver: Starting...
Apr 24 21:05:21 ubuntu1 AMBEserver[87711]: AMBEserver: Opened serial port /dev/ttyUSB0 at 460800 >
john@ubuntu1:~$

but then fails:
john@ubuntu1:~$ sudo systemctl status ambeserver@...
● ambeserver@... - DV3000 Daemon
     Loaded: loaded (/lib/systemd/system/ambeserver@.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Fri 2020-04-24 21:05:55 EDT; 21s ago
    Process: 87701 ExecStartPre=/bin/chgrp dialout /dev/ttyUSB0 (code=exited, status=0/SUCCESS)
    Process: 87710 ExecStartPre=/bin/chmod 0660 /dev/ttyUSB0 (code=exited, status=0/SUCCESS)
    Process: 87711 ExecStart=/usr/sbin/AMBEserver -p ${UDP_PORT} -i /dev/ttyUSB0 $AMBESERVER_FLAG>
   Main PID: 87711 (code=exited, status=1/FAILURE)

Apr 24 21:05:21 ubuntu1 systemd[1]: Starting DV3000 Daemon...
Apr 24 21:05:21 ubuntu1 systemd[1]: Started DV3000 Daemon.
Apr 24 21:05:21 ubuntu1 AMBEserver[87711]: AMBEserver: Starting...
Apr 24 21:05:21 ubuntu1 AMBEserver[87711]: AMBEserver: Opened serial port /dev/ttyUSB0 at 460800 >
Apr 24 21:05:55 ubuntu1 AMBEserver[87711]: AMBEserver: Couldn't find start byte in serial data
Apr 24 21:05:55 ubuntu1 AMBEserver[87711]: AMBEserver: error receiving response to reset
Apr 24 21:05:55 ubuntu1 AMBEserver[87711]: AMBEserver: Could not initialize the DV3K!
Apr 24 21:05:55 ubuntu1 systemd[1]: ambeserver@...: Main process exited, code=exited,>
Apr 24 21:05:55 ubuntu1 systemd[1]: ambeserver@...: Failed with result 'exit-code'.
Apr 24 21:05:56 ubuntu1 systemd[1]: ambeserver@...: Triggering OnFailure= dependencie>
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