I'm currently at a customer site and am seeing the same behavior and I can say that the memory is definitely not running out. We are baffled as well, it happens regularly, but is also intermittent. Same search will complete some of the time, and then a more often than not fail.
Customer is running 7.1.4, and I don't see anything in the release notes that would explain this behavior. We are going to be opening a support ticket to look into it.
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