The crash seems to occur either:

1. Immediately following a click on 'close' in the pop-up Edit Index Entry form, or

2. on clicking the location of the index entry in the text.

The crash may also occur on saving the document, following an attempted edit of an index entry.

Does this also happen with a completely new document?

We would need an example document. If it is only with your book, you could try sanitizing it and then attaching it to the report.
https://wiki.documentfoundation.org/QA/Bugzilla/Sanitizing_Files_Before_Submission#Sanitize_file_text

Set to NEEDINFO.
Change back to UNCONFIRMED after you have provided the document.

Sanitizing according to instructions makes reproducing the bug impossible, since indexed items all look like the rest of the text (xxxxx....)

If it matters, the problem did not occur until the document was nearly complete (350pp) so wondering if it could be size/RAM related.

Ok, one thing that you could do is produce a debug trace of the crash and attach it here. In Linux you have to install a debug package first. All the instructions are here: https://wiki.documentfoundation.org/QA/BugReport/Debug_Information

Hi,
I can confirm this bug (version 5.0.4.2 M$ Windows 7). The document producing it has 293 pages of text plus 17 pages of index (100.363 words)

Confirm here, on Linux x64, LO 5.0.4.2 .deb package. It's happened twice now, on two unsaved Writer documents, both new, both very small for testing purposes only. 

When the Index Entry dialog is open, I will click around to see what options exist. Usually clicking on the "Key" dropdown box freezes not only LO, but the whole system, which has to be hard-shutdown. (The system itself is very stable.)

The first crash was when trying to redo an index entry, the second was on trying to create one afresh. Also, each time a popup help balloon was showing on the dialog.

This second time, on reboot LO automatically went into recover mode for the two documents that had been open, and it got stuck in a loop doing so, until I abandoned the recovery process.

Riccardo and Paul: could you try getting a trace of the crash like I mention in comment 5?
Not sure how to do it, if it is a freeze instead of a crash..

I upgraded to 5.1.0.3, still on Debian x64, and now there is no "edit index entry" in the context menu. Nor do I see it in the menus.

Clarify: I see no way to edit index entries on new documents. The ability to do so is present on existing documents with index entries.

(In reply to Paul from comment #9)
> I upgraded to 5.1.0.3, still on Debian x64, and now there is no "edit index
> entry" in the context menu. Nor do I see it in the menus.

I have the context menu item.

64-bit, KDE Plasma 5
Build ID: 5.1.1.3 Arch Linux build-2
CPU Threads: 8; OS Version: Linux 4.4; UI Render: default; 
Locale: fi-FI (fi_FI.UTF-8)

Dear Bug Submitter,

Please read this message in its entirety before proceeding.

Your bug report is being closed as INSUFFICIENTDATA due to inactivity and
a lack of information which is needed in order to accurately
reproduce and confirm the problem. We encourage you to retest
your bug against the latest release. If the issue is still
present in the latest stable release, we need the following
information (please ignore any that you've already provided):

a) Provide details of your system including your operating
   system and the latest version of LibreOffice that you have
   confirmed the bug to be present

b) Provide easy to reproduce steps – the simpler the better

c) Provide any test case(s) which will help us confirm the problem

d) Provide screenshots of the problem if you think it might help

e) Read all comments and provide any requested information

Once all of this is done, please set the bug back to UNCONFIRMED
and we will attempt to reproduce the issue. Please do not:

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b) update the version field in the bug or any of the other details
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Warm Regards,
QA Team

MassPing-NeedInfo-20161108