Players cannot join

Your server is online, you can join, and a friend gets Failed to login: Invalid session or Failed to verify username and never gets in. The console shows nothing useful: on new versions one line saying the account could not be verified, on old versions only a lost connection.

If every player gets it at once, you included, Mojang’s login is probably down: see When Minecraft login is down.

What is happening

A Minecraft server in online mode asks Mojang to verify every account at login. A launcher without a Microsoft login (a “cracked” or offline launcher, or a player who is not signed in) cannot pass that check, so the server drops the login before the player ever loads the world.

FadeHost watches for those dropped logins. After a few of them in ten minutes the server page shows a Players could not log in banner with the names it saw, and the owner gets one alert a day if alerts are set up.

The two fixes

Both are one button on the banner and restart a running server:

Login protection (every server): the door in front of your server lets those players register a password and log in with it, while everyone with a Microsoft login keeps logging in as usual and the server stays in online mode. Names stay safe from impersonation. This is the same switch as Settings → Login protection.
Switch to offline mode (paid servers): switches the account check off entirely, so anyone can join under any name. Only do this on a whitelisted or private server. This is the same switch as Settings → Game → Online mode.

Free and trial servers stay in online mode, so they get the login protection button only.

If that is not the problem, Dismiss for a week hides the banner; it comes back if new dropped logins keep arriving after that.

Sometimes the problem is on Mojang’s side. When their login service is down, every player gets Failed to verify username or cannot sign in to the launcher at all, on every server that checks accounts with Mojang.

On any Minecraft Java server, the panel’s sidebar shows Minecraft login with Working, Slow or Down. Our servers check it every minute: Microsoft sign-in, the account check on join, skins and capes, and player names. Click the line to see each one and since when it has been that way.

While it reads slow or down:

Servers in online mode or with login protection show a note at the top of the server page, on a phone too.
The Players could not log in banner and its alert stay quiet, because the players it would name do have a Microsoft login.

Nothing on your server needs changing. Players get in again as soon as Mojang’s login recovers, and while it is only slow, a second try often works.