@hexparrot have commented issues like this on several occations, the lates one here:
How linux and Minecraft treats RAM has also been discussed here:
The first thing to remember is that how the minecraft servers them selves work, is not under MineOS and the WebUI’s control. MineOS and it’s WebUI only provides a frontend and a easy environment to mangae and run the servers. I think you would be better off asking a forum spesific to your server type for help with this. We will try our best to help you, but our knowledge of all the different servers, and the myriad of more or less homebrewed mods and plugins, is spotty at best.
A second thing to remember is that the larger a server is, the more memory it needs.
My best recomendation right now is to disable all mods, then see if the problem persists.
You can also try to start the server, then run the following commands in SSH:
You can both check ram and clear it with these commands:
free -m sync echo 3 > /proc/sys/vm/drop_caches free -m
This lists all memory (used and available) on your linux machine, then clears all used cache, then lists memory again. If the first command (free -m) do not reveal low memory I would not worry.
If all else fails: Add more memory, and let Minecraft have more memory to play with.