From Serenity Servers Support

Console Error: Java Heap Space. Log: java.lang.OutOfMemoryError: Java heap space

Unfortunately means your server is out of the amount of ram you have purchased. A quick fix is simply to restart the server, but for longer terms you may need to consider an upgrade.

Really? My server barely has players!
We allocate Java the amount of ram you as a client pay for, but since java is an engine. Both that, the server itself, plugins etc. All need ram, it may decrease the amount of players your server can hold.

Do i really need to upgrade?
No you do not need to upgrade, but if the error occur multiple times a day. It may be the only solution, either that or you need to limit player count and/or uninstall plugins.

Other fixes?
The easiest resolve would be to upgrade your servers RAM, however if you do not want to do this, you will need to do the following;
Use a smaller modpack like Bukkit or Vanilla
Restart your server more often
Run a smaller world file & limit the world size using world border plugin.
If you server continues to crash even after following this guide, please contact support or a custom quote.