Is it saying that during server startup or while the server has already been online for a while?
If it is saying that during server startup, did you downgrade your server version? If so, that is why. Once you upgrade the server version, you cannot downgrade it without a backup or resetting the world. You can also just go back to whatever version it was on before to fix it.
If it is saying that while the server has already been online for a while, you're server doesn't have enough ram to do what it is doing to run properly.