[KevinH]

Your error message says its an error reading from the sigil.ini file. So it got corrupted.
On a standard macOS install, the sigil.ini file is located here:

"/Users/USERNAME/Library/Application Support/sigil-ebook/sigil/sigil.ini"

Where you replace USERNAME with your username. Note the space in "Application Support"

If you do not see a Library folder in your home directory (for some silly reason Apple started hiding it) use the Go to menu to get there.

Or alternatively, you can use Terminal.app to delete that file.

Just ask if you would prefer that approach and I can lead you through the terminal commands.