From the attached crash reports it looks like the server can use up more than 1GB of RAM with only two players playing.
First crash happened when the second player tried to enter the Nether, first was already there. Second crash happened after respawning the dragon if I remember correctly, and the rest occurred during end exploration.
Some of the crashes were triggered by watchdog, after the server being unresponsive (it took quite a while to stop it). I'm sorry if they're unrelated.
I raised the limit to 2GB, will update the issue if it crashes again.
Environment
OS: Debian GNU/Linux 7 (wheezy)
Java: OpenJDK, version 1.7.0_79
Duplicate of MC-12949 – If you have not, please use the search function in the future, to see if your bug has already been submitted.
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