java.lang.IndexOutOfBoundsException: Index 3 out of bounds for length 1
...
at com.xpn.xwiki.doc.XWikiDocument.updateDocumentSection(XWikiDocument.java:8553)

This shows that you were editing a section of a wiki page. This is quite an important detail. The save failed here because:

• you started editing the 4th section of the page
• by the time you saved there was only a single section in that page, so the section 4 you were editing couldn't be saved

The question is how did that page go from 4 sections (at least) to only 1 section? There's nothing in your description, logs and screenshots to explain this. Note that the realtime editing code has changed a lot since 14.10.16. On 16.2.0 the in-place edit mode supports realtime so by default so you won't be editing a single section anymore (but the entire page). But even if I disable the in-place edit mode, I still can't reproduce the problem in 16.2.0 with standalone WYSIWYG editor and realtime if I edit a single section of a page.

Best would be if you can reproduce the issue on a more recent version of XWiki (like 16.2.0) or give us more detailed steps to reproduce the problem. Otherwise, it's hard to understand why you lost those sections while editing in realtime.