This has happened suddenly. My most recent activity (other than some minor changes to some content) was a number of changes in my CSS file. But this passes the CSS tests.
I click on Reports, see the progress bar start to run in its dialog, and then a dialog pops up that says "Sigil has encountered a problem. Sigil may need to close." Show Details reveals:
Error info: Unknown exception
Sigil version: 0.9.10
Runtime Qt: 5.6.2
Compiled Qt: 5.6.2
Platform: Windows Sysinfo ID 192
This also happens on 0.9.9.
I'm running Win 10 Professional (64 bit).