@Zminer777 OK it seems to be a connection issue afterall. The miner is using a "ethernet over power lines" adapter and while I was on site today, the connection really dropped for like ~5 seconds due to presumably power line noise (miners pull a lot of electricity).

This resulted in the miner stopping (I can see this by looking at the GPU fans, they stop spinning after a while when disconnected from pool).

I connected via RDP and violla - it started mining again.

I think this is a bug in GMiner, when launched via RDP. I think when it releases the GPUs, it's unable to use them again because of windows reasons presumably because "there's nobody at the computer, so why would any program want to use the GPU". When I connect, it starts up instantly. Is there anything we can do about this?