@mardah Hi! Home assistant sends a request for gateway version every 10 seconds and if the gateway doesn't respond with version within another 10 seconds, home assistant disconnects from the gateway and tries to reconnect. Have you monitored your gateway serial port during these tests? Seeing that side of the conversation would be interesting. The gateway not responding to the version request is a sign that something is not working properly with the gateway, or that it is unstable. Once I had this issue when I had built the gateway sketch with an avr version that had a bug, and caused the gateway to reboot when starting the communication. This post describes what version of avr had the bug: https://forum.mysensors.org/topic/6257/ethernet-gateway-home-assitant/4

I found the problem: https://github.com/LowPowerLab/SPIFlash/issues/16 The sketch I used to verify the SPI Flash was missing a SPI.begin() in setup().

I soldered just the two caps and an MCP1703 on a new board and have not manage to fry it. So maybe there is another cause. I'm suspecting the WS2812, it's new on my latest batch of sensor boards. But I don't understand how it could destroy the voltage regulator or ATMega. Maybe something is wrong with the PCB layout causing some unwanted currents?

Chemistry. I have no idea. Initial voltage increase is common in e.g. AAs if you switch load type and/or let the them rest for a while. I often put used batteries from my kids toys in my sensors.