The Minecraft authentication servers are unreachable
What it looks like in your log
Authentication servers are down. Please try again later, sorry!
What causes it
The server cannot reach Mojang session services, so premium logins fail. Either Mojang is having an outage, or this machine cannot resolve or reach the internet.
How to fix it
Check the official Mojang status channels. If only this server is affected, test DNS and outbound HTTPS from the machine. Do not switch to offline mode as a fix; it removes authentication entirely.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log