I checked my event log: I do show a couple of those QT5 crashes in recent times
W7pro x64 8G RAM core2 duo
Today was a bad Sigil day

2 crashes while editing with no module named
Leading up to the crash:
Search and replace with the search getting slower after each replace next
there was nothing
unusual with the document that I had not done many times before

300K, ~8 HTML files I was pre splitting (placing a sigil split marker via REGEX)