Server crash problem [HELP REQUIRED]

Hello dear PlanetMinecrafters!

Since several days, my Minecraft server keeps crashing. CALM DOWN! I know what you're going to answer! "ERMAHGERD UPDATE YOUR PLUGINS"! But I'm answering NO, it's apparently not that. In fact, I have no idea why my server crashes. Actually it doesn't even crash; let me explain.

All my happy HopeCrafters are playing when suddenly BAM we can't use any command! However we can still chat. When I say we can't use any command, I mean that nothing happens when you use them, like if the server never received them (which is actually the case since they're not shown in the console). During several minutes after the beginning of that "freeze/crash" all we can do it chat. We can't see other players moving etc. Finally, after a few seconds/minutes of this weird lag, we are disconnected from the server and when we try to reconnect it says: "Can't reach server".

The most interesting part is that even the server doesn't know why it crashes; here is the log of the last crash:



As you can see, no one is online and everything is normal during 2 HOURS! The only thing done by the server is an auto-saving task every 10 minutes. There is no error, no command, no world edit abuse, NOTHING! It's incomprehensible!

It can't be a lack of RAM (the server has 6GB and barely use half of it), it can't be a crash caused by players (e.g world edit etc) and it is APPARENTLY not a plugin-error, so...

WHAT IS IT?

If you have any idea, leave a comment bellow! Thanks!