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I had some problems with similar error message,they occurred when I wanted to restart
osmocom-apps after having removed phone with its usb cable
from pc and then reinserting phone and usb-cable. But when I just remove the phone from
the usb-cable and then reconnect phone to the cable the "Cannot open serial device
/dev/ttyUSB0" messages no longer occur and one can then rerun the osmocom-apps
without error messages.
Regards
Erich