I tested something... 
I only connected a 10K resistor from RX to GND. Seems like the C2 always boots now. A 20K resistor seems to work too... 
Posts by LibreEd
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I still had the UART adapter connected (but not plugged into my PC).
Exactly the same problem here. My C2 (LibreElec 12) boots at most 1 or 2 times in 20 power-up attemps. When i connect a CP2102 UART adapter (only to the C2 is enough), it boots every time. Very odd...
