Server Crash

I host a private sever for my friends and my server crashed 3 times as I did the following:

I have under my farms (wheat farm and nether wart farm) a cow farm.
When trying to reproduce the cows with wheat after a few clicks on the cows the server simply crashes but the server does not close. I can save the server(map) and stop it with no problem. But everything just freezez and it stops working.

Then creates the crash report under the attachement of this thread.

I have to mention that this did not happen when I had less cows. Now there is a bunch and when I try to breed the cows (all at the same time) the server just freezes and creates the crash report.

After I stop the server with the stop command, I restart it and the cows I managed to breed worked(the breeding process worked).

You have a photo of how the farm looks and how many cows are there. I did not count them.

Good luck!