Assetto Corsa Competizione servers, despite being independent, require the KUNOS master server to be operational in order to register in the lobby. That is why there are websites like ACC STATUS where we can check the status of the KUNOS server.

There, we can see the status of the ACC master server over the last 60 days. Although it is usually reliable, sometimes before and after an update, it tends to have downtimes, so it can be very useful to ensure that the problem is global and not just yours.