If a player is in the end, the server crashes after 15 seconds and the crash reports that a tick took too long (usually in the 30 seconds). This did not ever happen in 14w29b or previous versions, and the setup for the server has not changed.
Related issues
duplicates
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kumasasa
migrated
I had a similar issue see MC-63728, with the log
Please attach the server log to MC-63590