Circuit Python launches with cruft at console leading to SyntaxError - ModemManager to blame

[cefn]

On (L)ubuntu 17.10 if I plug in a stock Feather M0 Express running CircuitPython provided by adafruit-circuitpython-feather_m0_express-2.2.3.uf2 and then wait a respectful amount of time for it to launch, connecting using screen /dev/ttyACM0 115200 then I get

SyntaxError: invalid syntax

Whatever I type as my first command, including empty line.

If I am more swift to connect to the board, I can see a whole load of AT commands are echoed to /dev/ACM0 sent as if the Feather was being configured as a modem. The echoing looks like this...

Auto-reload is on. Simply save files over USB to run them or enter REPL to disable.

Press any key to enter the REPL. Use CTRL-D to reload.
Auto-reload is on. Simply save files over USB to run them or enter REPL to disable.

Press any key to enter the REPL. Use CTRL-D to reload.

Adafruit CircuitPython 2.2.3 on 2018-02-06; Adafruit Feather M0 Express with samd21g18
>>> T
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'T' is AT
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'AT' is not defined
>>> ATTraceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'AT' is not defined
>>> ~x~

So my command is being added at the end of some modem initialisation string which is already in the buffer before I connect.

More investigation suggested it might be to do with the pantech plugin in modemmanager.

Running sudo apt remove modemmanager seemed to fix the issue.