OK.
Attached are the log files. I added another log to show the working, about to fail, and failed situation.
working_log.txt is a Sigil instance used and closed normally
about_to_crash_log.txt is a Sigil instance where it worked normally, but I set it up in a known failure mode eg. I closed it while it was on the secondary monitor
crash_log.txt is a Sigil instance that failed to open
edit: 
ooops, I think I may have forgotten to clear the log file between the working_log, and the about_to_crash_log... The first 166 lines of the about_to_crash_log are the same as the working_log.
Do you want me to redo?