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Modem manager in Ubuntu GNU/Linux gets in the way. This results in intermittent automatic data extraction. One usually sees that as "Port can't be opened". SerialException details say "Device or resource busy". PID of modem-manager can be found using sudo fuser /dev/ttyUSB0 when this happens.
sudo fuser /dev/ttyUSB0
Looks like gpsd is irrelevant since product id in this watch is 0xea61 and not 0xea60 which is affected according to /lib/udev/rules.d/40-gpsd.rules from gpsd.
See also debian bug.
This might be related to symptoms described in issue #24.
This closes #26