Mojang is rate limiting this server
What it looks like in your log
com.mojang.authlib.exceptions.AuthenticationException: The client has sent too many requests within a certain amount of time
What causes it
The server or a plugin is asking the Mojang API for profile data faster than allowed, common on shared hosting where one IP serves many servers.
How to fix it
Turn on profile caching in whichever plugin does name lookups, reduce lookup-heavy plugins, and wait; the limit resets on its own.
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