It is possible for Wacom CTL-x100 tablets to boot in Android mode (the mode they use when interfacing with android devices like phones) instead of PC mode in some rare cases.
To fix this, press the outer 2 express keys for 3-4 seconds until the LEDs change brightness.
This toggles the tablet’s operating mode between PC (high LED brightness) and Android mode (low LED brightness).
OpenTabletDriver can only talk to the tablet in PC mode.
I am still having problems
See General FAQ.