Could be all sorts of things - versions of Jira, Java, your database, unusual configurations, etc. I'm afraid we can't really tell you much without more information.
What does the log say too? Even if it's just sitting there writing nothing, that might trigger someone to say "I've seen that, have you looked at ...."
I'd also skim through the "related questions" on the right here - there's at least one in there that has had a similar problem and found a fix.
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