I haven't been able to reproduce this issue, yet. If it happens again, please warn me.
I suspect that it could have been caused by an out-of-memory error. To be on the right side, I expanded the system memory.
P.S.: I will do some more tests when I will have more spare time (after installing all the asics). If it turns out to be an OOM problem, I will try to refund it by paying to the pyramining reward as many BTC as they would have been earned from a solved block in these days (around 5BTC). I am sorry but errors may happen and I hope that this kind of refund will be enough.