Failed to verify username! What it means, and the fix

1What happened when the player joined

With online-mode=true, the default, the player's game tells Mojang's session server it's joining this server, and the server then asks Mojang whether it did. On 29 Sep we joined a Paper 1.21.1 server in online mode with a test client that doesn't sign in. The client was turned away, and the server logged:

The player's screen says Failed to verify username!, in their game's language. So the server did reach Mojang, and Mojang had no record of that player joining.

2Not to be confused with servers down

We then blocked the same server's outgoing HTTPS, the way it asks Mojang, and joined again. The message changed:

That one is about the server's connection to Mojang, or Mojang's servers being down, not the player's account. A firewall on the server that blocks outgoing HTTPS causes it, as ours did.

3What fixes it

• The player signs in properly. Online mode admits accounts that own Minecraft: Java Edition, signed in through the Minecraft Launcher or a launcher that signs in with Microsoft, like Prism Launcher. A launcher that plays offline, without signing in, can't pass the check.
• Turning online mode off is not the fix. With online-mode=false the server skips the check and believes whatever name a game sends, so anyone can join as anyone, your operators included. Keep it on, and keep an allowlist.
• Authentication servers are down: check the server's connection. Let it make outgoing HTTPS connections, and wait out an outage on Mojang's side.

4On a Playkeeper server

• Online mode is always on, and the allowlist is enforced from the first start. Playkeeper never takes online-mode from a modpack's settings either.
• Friends get an invite link from the Players tab, or you add their Minecraft name there.
• Bedrock friends join a Paper or Purpur server with Bedrock players on, signed in with their Xbox account, and go on the allowlist as . and their gamertag.