ERROR: java.lang.OutOfMemory

This error unfortunately means your server is out of memory (RAM), there are several options to resolve this issue and what could have caused it.

The easiest way to resolve this would be to upgrade your servers RAM. However if you do not want to do this, you will need to do the following..

1. First, if you don't often restart your server. Restart the server daily
2. Update all plugins, some may have memory leaks
3. Change modpack to something more "light" such as Tekkit classic, Tekkit Lite, Bytesize pack etc
4. Remove some mods / plugins from the server
5. If you run multiple world (other than the nether / end dimensions) try removing some that are not so essential.
6. Finally, you can always upgrade your server. You can do this easily by clicking the drop down to the right of the restart button and selecting upgrade server.

If you server continues to crash even after following this guide, please contact support.