This has been happening for many LibreOffice versions on many different computers. Most recent is:
LibreOffice Version: 5.1.2.2 (x64)
Build ID: d3bf12ecb743fc0d20e0be0c58ca359301eb705f
CPU Threads: 8; OS Version: Windows 6.1; UI Render: GL;
Locale: en-US (en_US)
AKA Windows 7 x64
Is it related to the built in HSQLDB timing out or something?
EDIT: This seems to be related to this bug:
https://bugs.documentfoundation.org/show_bug.cgi?id=98048
Add yourself to the CC list of that bug if you are having the same problem. Hopefully it’s something that can get fixed…