The server could not create any more threads
What it looks like in your log
java.lang.OutOfMemoryError: unable to create native thread: possibly out of memory or process/resource limits reached
What causes it
The process hit the thread limit of its container or user account, not a lack of RAM.
How to fix it
Raise the process/thread ulimit (or the container pids limit). If a plugin or mod spawns threads without bound, the stack trace names it; remove it.
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