Hello @alessio.dapelo
I did a quick google search for Notification condition [NO_MASTER] has been fixed.
. I came across several post on this issue and it seams a couple different methods of resolving it. Please take a look at below and see if they help resolved your issue. If not, please post your full Graylog and elasticsearch log file using the markdown. Thanks
Periodic "Notification condition [NO_MASTER] has been fixed.