Usually, USB keys get assigned a new port such as /dev/ttyACM0, /dev/ttyACM1, etc..., each time they are unplugged and replugged into the RPI or if the RPI reboots. A solution consists in making these ports permanent via symlinks.
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This "feature" is a problem as the specified device may not be recognized anymore by configured softwares, like the Z-Wave Binding or openHab.
See a definitive solution here: make serial USB ports persistent via symlinks.