millis() has a rollover after approximately 50 days. Could that be something?

Under Development is a great thread about security:

[security] Introducing signing support to MySensors

An example is the radio module. The traces are super close to the pads, or are they even touching some of the pads? But I like the form factor of the card!

No no, this is not your final board. The traces are routed really close to each other.