It's not associated with your "Reports dialog open" bugs. That's some good news.
The other good news is that it's largely my fault -- partly for trying to be too clever within certain confines of CSS/HTML/XML.
It turns out that one of my HTML files in the book is something I've been using as a "template" for creating a Chapter file for the book. I recently cleaned up and improved all the CSS (yuck!) for that since I use it to auto-generate chapter and section numbering -- but that's largely irrelevant. Except that when I did that I added a highly visible line in a <style> section in that HTML "template" file -- intentionally ill-formed CSS -- to remind me (if I missed it) that I had to set the chapter counter at that point in the HTML file. Sigil, in general, doesn't seem to care about this much, and just in Book View displays the red "This page contains the following errors" box -- which was precisely my goal to avoid overlooking setting that counter.
But apparently this drives reports CRAZY!. It throws an exception (I guess, who can blame it).
So I think that I can say I found the problem and it is I.

I guess you might want to catch that exception a little better. Otherwise, Sigil just crashes if you have an error in one of your content files and run Reports on it. You can't really ignore it in Reports since it's an error in the HTML file. But probably not a real high priority.