A login arrived with an impossible username
What it looks like in your log
The received string length is longer than maximum allowed (25 > 16)
What causes it
A username longer than 16 characters reached the server: a cracked launcher using an email address, broken proxy forwarding, or a nickname plugin misbehaving.
How to fix it
If you run a proxy, check the forwarding mode matches on both sides. Otherwise the player needs to log in with a real Minecraft account name.
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